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Adobe® Flex® 4
Training from the Source
Michael Labriola Jeff Tapper Matthew Boles Introduction by Matt Chotin, Flex Product Manager From the Library of Wow! eBook
Adobe® Flex® 4: Training from the Source Michael Labriola/Jeff Tapper/Matthew Boles Adobe Press books are published by: Peachpit 1249 Eighth Street Berkeley, CA 94710 510/524-2178 800/283-9444 For the latest on Adobe Press books, go to www.adobepress.com. To report errors, please send a note to
[email protected] Peachpit is a division of Pearson Education Copyright © 2010 by Michael Labriola and Jeffrey Tapper Adobe Press Editor: Victor Gavenda Project Editor: Nancy Peterson Editor: Robyn G. Thomas Technical Editor: Simeon Bateman Production Coordinator: Becky Winter Copy Editors: Karen Seriguchi, Darren Meiss, and Liz Welch Compositor: Danielle Foster Indexer: Karin Arrigoni Cover Design: Peachpit Press
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Trademarks Adobe, the Adobe logo, Flash, Flash Builder, Flex, Flex Builder, and LiveCycle are either registered trademarks or trademarks of Adobe Systems Incorporated in the United States and/or other countries. ActiveX and Windows are either registered trademarks or trademarks of Microsoft Corporation in the United States and/or other countries. Apple and Macintosh are trademarks of Apple Inc., registered in the United States and other countries. Linux is a registered trademark of Linus Torvalds. Microsoft and Windows are either registered trademarks or trademarks of Microsoft Corporation in the United States and/or other countries. Solaris is a registered trademark or trademark of Sun Microsystems, Inc. in the United States and other countries. All other trademarks are the property of their respective owners. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and Peachpit was aware of a trademark claim, the designations appear as requested by the owner of the trademark. All other product names and services identified throughout this book are used in editorial fashion only and for the benefit of such companies with no intention of infringement of the trademark. No such use, or the use of any trade name, is intended to convey endorsement or other affiliation with this book. Adobe Systems Incorporated, 345 Park Avenue, San Jose, CA 95110-2704, USA.
Notice of Liability The information in this book is distributed on an “as is” basis, without warranty. While every precaution has been taken in the preparation of the book, neither the authors, Adobe Systems, Inc., nor the publisher shall have any liability to any person or entity with respect to any loss or damage caused or alleged to be caused directly or indirectly by the instructions contained in this book or by the computer software and hardware products described in it. Printed and bound in the United States of America ISBN 13: 978-0-321-66050-3 ISBN 10: 0-321-66050-1 9 8 7 6 5 4 3 2 1
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To my wife Laura and my daughter Lilia; you both make me smile. —Michael Labriola My efforts on this book are dedicated to my wife Lisa and children Kaliope and Kagan. Without you to inspire me, this just wouldn’t be possible. —Jeff Tapper To Sandra, my wife, who has made the last 24 years together a joy. And to Scrappy, my furry fishing buddy. —Matthew Boles
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Bios Michael Labriola is a Founding Partner and Senior Consultant at Digital Primates. He has been developing Internet applications since 1995 and has been working with Flex since its 1.0 beta program. Michael is an Adobe Certified Instructor, Community Professional, Flex Developer Champion, and international speaker on Flex and AIR topics who has consulted for many of the world’s most recognized brands. At Digital Primates, Michael mentors client development teams using emerging technologies. At home, he spends his free time escaping from technology through wine and food. Jeff Tapper is a Founding Partner and Senior Consultant at Digital Primates. He has been developing Internet-based applications since 1995 for a myriad of clients, including Major League Baseball, ESPN, Morgan Stanley, Conde Nast, IBM, Dow Jones, American Express, Verizon, and many others. He has been developing Flex applications since the earliest days of Flex 1. As an instructor, Jeff is certified to teach all of Adobe’s courses on Flex, AIR, Flash, and ColdFusion development. He is also a frequent speaker at Adobe Development Conferences and user groups. Digital Primates provides expert guidance on rich Internet application development and empowers clients through mentoring. Matthew Boles is a Technical Training Specialist for the Adobe Technical Marketing group, and has been developing and teaching courses on Flex since the 1.0 release. Matthew has a diverse background in web development, computer networking, and teaching. He is coauthor of previous versions of this book, as well as a contributing author of the Adobe authorized Flex courseware.
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Acknowledgments Thanks to Jeff, Matt, and Simeon for their work and dedication on this book. Thanks to Chris Gieger for his gratis design work on the FlexGrocer application. A huge thank you to Robyn for her unending patience and diligence. My thanks to Victor and Nancy for their work on our behalf at Peachpit. Thanks to all of the team members at Digital Primates who picked up the slack when this book always took more time than expected. Thanks to my clients for the interesting work and inspiration to keep learning these technologies. And, as always, continuing thanks to Matt for dragging me into this adventure. Most of all, thanks to my wife Laura, who handles everything else without complaint or mention and is the real reason I accomplish anything at all. —Michael Labriola I would like to thank Mike, Matt, Sim, and Robyn for all their hard work, which has helped shape this book. Thanks to Chris Gieger for providing some design love for our application— Chris, sorry we couldn’t fully implement your excellent design. Special thanks go to the team at Adobe who has made this all possible, especially the efforts of Matt Chotin and Deepa Subramaniam. Thanks to the editorial staff at Adobe Press, who was faced with the Herculean task of making our writing intelligible. —Jeff Tapper Thanks to Jeff, Mike, Robyn, and Simeon for the great work getting this book out. —Matthew Boles
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Contents Foreword
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Introduction Lesson 1
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Understanding Rich Internet Applications
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The Evolution of Computer Applications . . . . The Break from Page-Based Architecture . . . . The Advantages of Rich Internet Applications RIA Technologies . . . . . . . . . . . . . . . . . . .
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Training from the Source
Lesson 5
Handling Events
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Breaking the Application into Components Introducing MXML Components . . . . . . . . . . . . . Splitting Off the ShoppingView Component . . . . . Breaking Out a ProductItem Component . . . . . . . Creating Components to Manage Loading the Data
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Contents
Lesson 11
Creating and Dispatching Events
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Understanding the Benefits of Loose Coupling . . Dispatching Events . . . . . . . . . . . . . . . . . . . . Declaring Events for a Component . . . . . . . . . . Identifying the Need for Custom Event Classes . . Building and Using the UserAcknowledgeEvent . Understanding Event Flow and Event Bubbling . Creating and Using the ProductEvent Class . . . .
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Using Formatters and Validators Introducing Formatters and Validators Using Formatter Classes . . . . . . . . . Examining Two-Way Bindings . . . . . . Using Validator Classes . . . . . . . . . .
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Training from the Source
Lesson 17
Customizing a Flex Application with Skins
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Creating Custom ActionScript Components Introducing Components with ActionScript 3 .0 Building Components Can Be Complex . . . . . . Understanding Flex Components . . . . . . . . . Why Make Components? . . . . . . . . . . . . . . . Defining a Component . . . . . . . . . . . . . . . . Creating the Visuals . . . . . . . . . . . . . . . . . . Adding Functionality to the Component . . . . . Creating a Renderer for the Skin . . . . . . . . . .
Appendix A
Setup Instructions
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Index
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Foreword Ten years ago Macromedia coined the term rich Internet application, or RIA, to describe the modern web application: one where a significant amount of data and business logic live on a server or in the cloud, but where the computing power of the desktop is leveraged to provide a great user experience. Flex has been at the center of the RIA landscape since Macromedia introduced it in 2004 and subsequent releases came under the Adobe name in 2006, after Adobe’s acquisition of Macromedia. With the release of Flex 4, Adobe is bringing the power of the RIA to an even broader audience of developers. The book you have in your hand is a great first step in learning to use that power. Adobe Flex comprises a number of elements. It has a declarative markup language called MXML to help you structure your application, and it uses ActionScript 3.0 (an implementation of ECMAScript) to add all the programming power you need. Your UI can be customized using the familiar CSS syntax. In addition to learning the languages that Flex uses (and when to use each), you’ll learn about the powerful component library and the best way to leverage it in your applications. Flex provides layout containers, form controls, formatters and validators, a rich text library, an effects and animation library, and much more to allow you to quickly build your user interface. And when Flex doesn’t provide something out of the box, you can easily build it yourself by extending what does exist. Much of our time in Flex 4 was spent introducing the next generation of the Flex component framework called Spark. Building on top of Flex’s existing architecture, Spark provides a much more expressive mechanism for developers and designers to work together on the appearance of their Flex applications. Spark promotes thinking in terms of Model-View-Controller (MVC) and enables the functionality of components to be cleanly separated from their visual appearance and behavior. In addition to simply making Flex applications easier to develop and maintain, this separation also allows for better collaboration between designers and developers, who may not be able to work on the application using the same tools. Of course it’s not enough to have a pretty interface; your application needs to be functional, and often that means manipulating data. You’ll find that Flex offers a variety of ways to connect to your backend data sources, from XML over HTTP, to SOAP web services, to an efficient remoting protocol called Action Message Format, or AMF, which is supported by every major backend technology. Flex also offers tight integration with Adobe LiveCycle Data
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Training from the Source
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Services, a powerful offering that makes it easy to manage large sets of data, especially when that data is shared among many users. While every element of Flex can be coded by hand with your favorite text editor on top of the open source Flex SDK, Adobe Flash Builder 4 is a fantastic IDE built on top of Eclipse that can help build and test a lot of your functionality faster. And as part of Flex 4, Adobe is introducing a new tool, Adobe Flash Catalyst, which allows designers to easily collaborate with developers in creating great user experiences. Additionally, there are a number of third-party tools, libraries, and extensions (some written by your authors!) aimed at making you more productive in your development. But it’s not enough to simply know about the pieces that make up a Flex application. You have to know how to use them well. Adobe Flex 4: Training from the Source draws from the expertise of its authors to present a number of lessons that will not only introduce you to the concepts of Flex, but also help you use best practices as you go. With this introduction you’ll find yourself quickly building applications that look better and do more than anything you’ve done before. You know those applications that you see in the movies that seem so unrealistic? With the power of Flex 4 and its related tools, they may not be that far off! We at Adobe can’t wait to see what you build. Matt Chotin Senior Product Manager Adobe Systems, Inc.
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Introduction In March 2002, Macromedia coined the term rich Internet application. Back then, the idea felt somewhat futuristic, but all that has changed. Rich Internet applications (RIAs) are today’s reality. Macromedia introduced Flex in 2004 so that developers could write web applications for the nearly ubiquitous Flash platform. These applications benefited from the improved design, usability, and portability that Flex made possible, dramatically changing the user experience. These features are a cornerstone of Web 2.0, a new generation of Internet applications focused on creativity and collaboration. Since the introduction of Flex, Macromedia—and now Adobe—has released versions 1.5, 2, 3, and 4 of Flex. With each subsequent version, creating rich, compelling, intuitive applications has gotten easier, and the bar has been raised on users’ expectations of web applications. Countless organizations have discovered the benefits of Flex and have built and deployed applications that run on the Flash platform. But Flex 1 and 1.5 were most definitely not mass market products. The pricing, lack of IDE, limited deployment options, and other factors meant that those early versions of Flex were targeted specifically for large and complex applications as well as for sophisticated developers and development. However, with the new releases of the Flex product line, all this has changed. Flex 2 was released in 2006 and made Flex development a possibility for many more people, as it included a free software development kit (SDK). With the open sourcing of Flex 3, and the announcement of free versions of Flash Builder for students, Flex development is within the grasp of any developer with enough foresight to reach for it. The release of Flex 4 has made it even easier to build rich, efficient, cutting-edge applications. Among the many improvements of Flex 4 is the streamlining of the workflow between designer and developer, greatly easing the process of bringing intuitive, compelling designs to even more Flex applications. Getting started with Flex is easy. Flex itself is composed of two languages: MXML, an XMLbased markup language; and ActionScript, the language of Flash Player. MXML tags are easy to learn (especially when Flash Builder writes them for you). ActionScript has a steeper learning curve, but developers with prior programming and scripting experience will pick it up easily. But there is more to Flex development than MXML and ActionScript.
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To be a successful Flex developer, you will need to understand a number of concepts, including the following: • How Flex applications should be built (and how they should not) • What the relationships between MXML and ActionScript are, and when to use each • How to load data into a Flex application • How to use the Flex components, and how to write your own • What the performance implications are of the code you write • Which practices you should employ to write code that is scalable, manageable, and reusable Developing these skills is where this book comes in. As the authors, we have distilled our hard-earned Flex expertise into a series of lessons that will jump-start your own Flex development. Starting with the basics, and then incrementally introducing additional functionality and know-how, the author team guides your journey into the exciting world of RIAs, ensuring success every step of the way. Flex is powerful, highly capable, fun, and incredibly addictive. And Adobe Flex 4: Training from the Source is the ideal tour guide on your journey to the next generation of application development. Adobe Flex 4: Training from the Source is an update to the popular Adobe Flex 3: Training from the Source. It is our sincere intention that readers of the earlier book, as well those who are first exploring Flex with this book, will find this content compelling. Since the release of our previous book, the Flex SDK has been completely reworked. Among the many improvements are: • Much greater efficiencies at run time from Flex applications • A new component model that removes a lot of unnecessary code from applications, allowing them to be much smaller • A greatly improved designer and developer workflow • And much more It’s an incredible time to be an RIA developer, and we hope that this book provides you with all the tools you need to get started with Flex.
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Introduction
Prerequisites To make the most of this book, you should at the very least understand web terminology. This book isn’t designed to teach you anything more than Flex, so the better your understanding of the World Wide Web, the better off you’ll be. This book is written assuming that you are comfortable working with programming languages and that you are probably working with a server-side language such as Java, .NET, PHP, or ColdFusion. Although knowledge of serverside technologies is not required to succeed with this book, we invoke many comparisons and analogies to server-side web programming. This book is not intended as an introduction to programming or as an introduction to object-oriented programming (OOP). Experience with OOP is not required, although if you have no programming experience at all, you might find the materials too advanced.
Outline As you’ll soon discover, this book mirrors real-world practices as much as possible. Where certain sections of the book depart from what would be considered a real-world practice, every attempt has been made to inform you. The exercises are designed to get you using the tools and the interface quickly so that you can begin to work on projects of your own with as smooth a transition as possible. This curriculum should take approximately 28–35 hours to complete and includes the following lessons: Lesson 1: Understanding Rich Internet Applications Lesson 2: Getting Started Lesson 3: Laying Out the Interface Lesson 4: Using Simple Controls Lesson 5: Handling Events Lesson 6: Using Remote XML Data Lesson 7: Creating Classes Lesson 8: Using Data Binding and Collections Lesson 9: Breaking the Application into Components Lesson 10: Using DataGroups and Lists
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Training from the Source
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Lesson 11: Creating and Dispatching Events Lesson 12: Using DataGrids and Item Renderers Lesson 13: Using Drag and Drop Lesson 14: Implementing Navigation Lesson 15: Using Formatters and Validators Lesson 16: Customizing a Flex Application with Styles Lesson 17: Customizing a Flex Application with Skins Lesson 18: Creating Custom ActionScript Components
Who Is This Book For? All the content of this book should work well for users of Flash Builder on any of its supported platforms.
The Project Application Adobe Flex 4: Training from the Source includes many comprehensive tutorials designed to show you how to create a complete application using Flex 4. This application is an online grocery store that displays data and images and takes a user through the checkout process, ending just before the data would be submitted to a server. By the end of the book, you will have built the entire application using Flex. You will begin by learning the fundamentals of Flex and understanding how you can use Flash Builder in developing the application. In the early lessons, you will use Design mode to begin laying out the application, but as you progress through the book and become more comfortable with the languages used by Flex, you will spend more and more time working in Source mode, which gives you the full freedom and flexibility of directly working with code. By the end of the book, you should be fully comfortable working with the Flex languages and may even be able to work without Flash Builder by using the open source Flex SDK and its command-line compiler.
Errata Although we have made every effort to create a flawless application and book, occasionally we or our readers find problems. The errata for the book will be posted at www.flexgrocer.com.
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Introduction
Standard Elements in the Book Each lesson in this book begins by outlining the major focus of the lesson at hand and introducing new features. Learning objectives and the approximate time needed to complete all the exercises are also listed at the beginning of each lesson. The projects are divided into exercises that demonstrate the importance of each skill. Every lesson builds on the concepts and techniques learned in the previous lessons. Tip: An alternative way to perform a task or a suggestion to consider when applying the skills you are learning .
NoTe: Additional background information to expand your knowledge, or advanced techniques you can explore to further develop your skills .
cauTioN! Information warning you of a situation you might encounter that could cause errors, problems, or unexpected results .
Boldface text: Words that appear in boldface are terms that you must type while working through the steps in the lessons. Boldface code: Lines of code that appear in boldface within code blocks help you easily identify changes in the block to be made in a specific exercise step.
Code in text: Code or keywords appear slightly different from the rest of the text so you can identify them. Code block: To help you easily identify ActionScript, XML, and HTML code within the book, the code has been styled in a special font that’s different from the rest of the text. Single lines of ActionScript code that are longer than the margins of the page are wrapped to the next line. They are designated by an arrow at the beginning of the continuation of a broken line and are indented under the line from which they continue. For example: public function Product (_catID:Number, _prodName:String, ➥_unitID:Number,_cost:Number, _listPrice:Number, ➥_description:String,_isOrganic:Boolean,_isLowFat:Boolean, ➥_imageName:String)
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Italicized text: Italics are used to show emphasis or to introduce new vocabulary. Italics are also used for placeholders, which indicate that a name or entry may change depending on your situation. For example, in the path driveroot:/flex4tfs/flexgrocer, you would substitute the actual name of your root drive for the placeholder. Menu commands and keyboard shortcuts: There are often multiple ways to perform the same task in Flash Builder. The different options will be pointed out in each lesson. Menu commands are shown with angle brackets between the menu names and commands: Menu > Command > Subcommand. Keyboard shortcuts are shown with a plus sign between the names of keys to indicate that you should press the keys simultaneously; for example, Shift+Tab means that you should press the Shift and Tab keys at the same time. CD-ROM: The CD-ROM included with this book includes all the media files, starting files, and completed projects for each of the lessons in the book. These files are located in the start and complete directories. Lesson 1, “Understanding Rich Internet Applications,” does not include exercises. If you need to return to the original source material at any point, you can restore the FlexGrocer project. Some lessons include an intermediate directory, which contains files in various stages of development in the lesson. Other lessons may include an independent directory, which is used for small projects intended to illustrate a specific point or exercise without impacting the FlexGrocer project directly. Anytime you want to reference one of the files being built in a lesson to verify that you are correctly executing the steps in the exercises, you will find the files organized on the CD-ROM under the corresponding lesson. For example, the files for Lesson 4 are located on the CD-ROM in the Lesson04 folder, in a project named FlexGrocer.fxp. The directory structure of the lessons you will be working with is as follows:
Directory structure
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Adobe Training from the Source The Adobe Training from the Source and Adobe Advanced Training from the Source series are developed in association with Adobe and reviewed by the product support teams. Ideal for active learners, the books in the Training from the Source series offer hands-on instruction designed to provide you with a solid grounding in the program’s fundamentals. If you learn best by doing, this is the series for you. Each Training from the Source title contains hours of instruction on Adobe software products. They are designed to teach the techniques that you need to create sophisticated professional-level projects. Each book includes a CD-ROM that contains all the files used in the lessons, completed projects for comparison, and more.
What You Will Learn You will develop the skills you need to create and maintain your own Flex applications as you work through these lessons.
By the end of the book, you will be able to: • Use Flash Builder to build Flex applications • Understand MXML, ActionScript 3.0, and the interactions of the two • Work with complex sets of data • Load data using XML • Handle events to allow interactivity in an application • Create your own event classes • Create your own components, either in MXML or ActionScript 3.0 • Apply styles and skins to customize the look and feel of an application • And much more . . .
Minimum System Requirements Windows • 2 GHz or faster processor • 1 GB of RAM (2 GB recommended)
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• Microsoft Windows XP with Service Pack 3, Windows Vista Ultimate or Enterprise (32 or 64 bit running in 32-bit mode), Windows Server 2008 (32 bit), or Windows 7 (32 or 64 bit running in 32-bit mode) • 1 GB of available hard-disk space • Java Virtual Machine (32 bit): IBM JRE 1.5, Sun JRE 1.5, IBM JRE 1.6, or Sun JRE 1.6 • 1024x768 display (1280x800 recommended) with 16-bit video card • Flash Player 10 or later
Macintosh • Intel processor based Mac • OS X 10.5.6 (Leopard) or 10.6 (Snow Leopard) • 1 GB of RAM (2 GB recommended) • 1 GB of available hard-disk space • Java Virtual Machine (32 bit): JRE 1.5 or 1.6 • 1024x768 display (1280x800 recommended) with 16-bit video card • Flash Player 10 or later The Flex line of products is extremely exciting, and we’re waiting to be amazed by what you will do with it. With a strong foundation in Flex, you can expand your set of skills quickly. Flex is not difficult to use for anyone with programming experience. With a little bit of initiative and effort, you can fly through the following lessons and be building your own custom applications and sites in no time.
Additional Resources Flex Community Help Flex Community Help brings together active Flex users, Adobe product team members, authors, and experts to give you the most useful, relevant, and up-to-date information about Flex. Whether you’re looking for a code sample or an answer to a problem, have a question about the software, or want to share a useful tip or recipe, you’ll benefit from Community Help. Search results will show you not only content from Adobe, but also from the community.
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Introduction
With Adobe Community Help you can: • Fine-tune your search results with filters that let you narrow your results to just Adobe content, community content, just the ActionScript Language Reference, or even code samples. • Download core Adobe Help and ActionScript Language Reference content for offline viewing via the new Community Help AIR application. • See what the community thinks is the best, most valuable content via ratings and comments. • Share your expertise with others and find out what experts have to say about using your favorite Adobe product.
Community Help AIR Application If you have installed Flex 4 or any Adobe CS5 product, then you already have the Community Help application. This companion application lets you search and browse Adobe and community content, plus you can comment and rate on any article just like you would in the browser. However, you can also download Adobe Help and reference content for use offline. You can also subscribe to new content updates (which can be automatically downloaded) so that you’ll always have the most up-to-date content for your Adobe product at all times. You can download the application from http://www.adobe.com/support/chc/index.html.
Community Participation Adobe content is updated based on community feedback and contributions: You can contribute content to Community Help in several ways: add comments to content or forums, including links to web content; publish your own content via the Community Publishing System; or contribute Cookbook Recipes. Find out how to contribute. http://www.adobe.com/community/publishing/download.html
Community Moderation and Rewards More than 150 community experts moderate comments and reward other users for helpful contributions. Contributors get points: 5 points for small stuff like finding typos or awkward wording, up to 200 points for more significant contributions like long tutorials, examples, cookbook recipes, or Developer Center articles. A user’s cumulative points are posted to their Adobe profile page and top contributors are called out on leader boards on the Help and Support pages, Cookbooks, and Forums. Find out more: www.adobe.com/community/publishing/community_help.html
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Frequently Asked Questions For answers to frequently asked questions about Community Help seehttp://community. adobe.com/help/profile/faq.html Adobe Flex and Flash Builder Help and Support www.adobe.com/support/flex/ where you can find and browse Help and Support content on adobe.com. Adobe TV http://tv.adobe.com is an online video resource for expert instruction and inspiration about Adobe products, including a How To channel to get you started with your product. Adobe Developer Connection www.adobe.com/devnet is your source for technical articles, code samples, and how-to videos that cover Adobe developer products and technologies. Cookbooks http://cookbooks.adobe.com/home is where you can find and share code recipes for Flex, ActionScript, AIR, and other developer products. Resources for educators www.adobe.com/education includes three free curriculums that use an integrated approach to teaching Adobe software and can be used to prepare for the Adobe Certified Associate exams. Also check out these useful links: Adobe Forums http://forums.adobe.com lets you tap into peer-to-peer discussions, questions and answers on Adobe products. Adobe Marketplace & Exchange www.adobe.com/cfusion/exchange is a central resource for finding tools, services, extensions, code samples and more to supplement and extend your Adobe products. Adobe Flex product home page www.adobe.com/products/flex Adobe Labs http://labs.adobe.com gives you access to early builds of cutting-edge technology, as well as forums where you can interact with both the Adobe development teams building that technology and other like-minded members of the community.
Adobe Certification The Adobe Certified program is designed to help Adobe customers and trainers improve and promote their product-proficiency skills. There are four levels of certification: • Adobe Certified Associate (ACA) • Adobe Certified Expert (ACE)
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• Adobe Certified Instructor (ACI) • Adobe Authorized Trainer (AATC) The Adobe Certified Associate (ACA) credential certifies that individuals have the entry-level skills to plan, design, build, and maintain effective communications using different forms of digital media. The Adobe Certified Expert program is a way for expert users to upgrade their credentials. You can use Adobe certification as a catalyst for getting a raise, finding a job, or promoting your expertise. If you are an ACE-level instructor, the Adobe Certified Instructor program takes your skills to the next level and gives you access to a wide range of Adobe resources. Adobe Authorized Training Centers offer instructor-led courses and training on Adobe products, employing only Adobe Certified Instructors. A directory of AATCs is available at http://partners.adobe.com. For information on the Adobe Certified program, visit www.adobe.com/support/certification/ main.html.
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From the Library of Wow! eBook
LESSON 1
What You Will Learn In this lesson, you will: • Explore alternatives to page-based architecture • See the benefits of rich Internet applications (RIAs) • Compare RIA technologies
Approximate Time This lesson takes approximately 30 minutes to complete.
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Lesson 1
Understanding Rich Internet Applications Computers have played a role in business environments for more than five decades. Throughout that time, the roles of client and server have constantly evolved. As businesses and their employees have become more comfortable delegating responsibilities to computers, the look, feel, and architecture of computerized business applications have changed to meet their demands. Today businesses are asking for even faster, lighter, and richer Internet applications. In this lesson, you will learn about this evolutionary environment and understand the business requirements that push developers to build rich Internet applications (RIAs).
You will use Flex to build the FlexGrocer application seen here.
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The Evolution of Computer Applications In the earliest days of computerized business applications, all the processing took place on mainframes, with the client having no role other than displaying information from the server and accepting user input. This setup was largely dictated by the high cost of processing power. Spreading powerful clients throughout the enterprise was simply not affordable, so all processing was consolidated and “dumb terminals” provided the user interaction. As memory and processing power became cheaper, dumb terminals were replaced by microcomputers (or personal computers). With the added power, more desktop applications, such as word processors and spreadsheets, could run as stand-alone applications, so no server was necessary. One challenge faced by organizations with microcomputers was a lack of centralized data. The mainframe era had everything centralized. The age of microcomputerdistributed data made it more difficult to centralize business rules and synchronize data across the enterprise. To help resolve this issue, several vendors released platforms that sought to combine the strengths of the microcomputer with those of the mainframe, which led to the birth of client/ server systems. These platforms afforded end users the power and ease of microcomputers while allowing business logic and data to be stored in, and accessed from, a centralized location. The new challenge introduced with client/server systems was distribution. Anytime changes needed to be made to client applications, IT departments had to manually reinstall or upgrade the software on every single desktop computer. Larger companies found they needed a full-time IT staff whose primary responsibility was keeping the software on the end users’ desktops current. With the explosive growth of the Internet in the 1990s, a new model for business applications became available. In this model, a web browser acted as a thin client whose primary job was to render HTML (Hypertext Markup Language) and to send requests back to an application server that dynamically composed and delivered pages to the client. This is often referred to as a “page-based architecture.” This model successfully solved the distribution problem of the client/server days; the specific page of the application was downloaded from the server each time an end user needed it, so updates could be made in a single centralized place and automatically distributed to the entire user base. This model was and continues to be successful for many applications; however, it also creates significant drawbacks and limitations. In reality, Internet applications bore a great resemblance to mainframe applications, in that all the processing was centralized at the server, and the client only rendered data and captured user feedback. The biggest problems with this surrounded the user interface (UI). Many of the conveniences that end users grew to accept over the previous decade were lost, and the UI was limited by the capabilities of HTML. For example, desktop software as well as client/server applications frequently allow drag-and-drop. However, HTML applications almost never do; they are prevented by the complexities of—and a lack of cross-browser
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support for—the DHTML (Dynamic HTML) elements that are required to implement dragand-drop in a pure HTML/DHTML solution. In most cases, the overall sophistication of the client-side solutions that could be built and delivered was greatly reduced. Although the web has offered great improvements in the ease and speed of deploying applications, the capabilities of web-based business applications took a big step backward because browser-based applications had to adapt to the limitations of the web architecture: HTML and the Hypertext Transport Protocol (HTTP). Today, the demands on Internet-based applications continue to grow and are often quite different from the demands of the mid-1990s. End users and businesses are demanding more from their investments in Internet technology. The capability to deliver true value to users is forcing many companies to look toward richer models for Internet applications—models that combine the media-rich power of the traditional desktop with the deployment and contentrich nature of web applications. As Internet applications begin to be used for core business functions, the maintainability of those applications becomes more crucial. The maintainability of an application is directly related to the application’s architecture. Sadly, many web applications were built with little thought about the principles of application architecture and are therefore difficult to maintain and extend. Today, it is easier to build a solid architecture for an application by providing a clean separation between the business, data access, and presentation areas. With the introduction of elements such as web services, the concept of a service-oriented architecture (SOA) has become more feasible for web-based applications. To meet the demands of businesses, RIAs should be able to do the following: • Provide an efficient, high-performance way to execute code, content, and communications. In the next section of this lesson, you will explore the limitations of the standard HTML-based applications and learn that traditional page-based architectures have a number of performance-related challenges. • Provide powerful and extensible object models to facilitate interactivity. Web browsers have progressed in recent years in their capability to support interactivity through the Document Object Model (DOM) via JavaScript and DHTML, but they still lack standardized crossplatform and cross-browser support. Although there are drafts of standards that may address these limitations in the future, building RIAs with these tools today so that they work on a variety of browsers and operating systems involves creating multiple versions of the same application. • Enable using server-side objects via Web Services or similar technologies. The promise of RIAs includes the capability to cleanly separate presentation logic and user interfaces from the application logic housed on the server.
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• Enable use of Internet applications when offline. As laptops and other portable devices continue to grow in popularity, one of the serious limitations of Internet applications is the requirement that the machine running the application be connected to the Internet. Although users can be online the vast majority of the time, business travelers know there are times when an Internet connection is not possible. A successful RIA should enable users to be productive with or without an active connection.
The Break from Page-Based Architecture For experienced web developers, one of the biggest challenges in building RIAs is breaking away from a page-based architecture. Traditional web applications are centered on the concept of a web page. Regardless of which server-side technologies (if any) are used, the flow goes something like this: 1. The user opens a browser and requests a page from a web server. 2. The web server receives the request. 3. The web server hands the request to an application server to dynamically assemble the web page, or it retrieves a static page from the file system. 4. The web server sends the page (dynamic or static) back to the browser. 5. The browser draws the page in place of whatever was previously displayed. Even when most of the content of the previous page is identical to the new page, the entire new page needs to be sent to the browser and rendered. This is one of the inefficiencies of traditional web applications: Each user interaction requires a new page to be loaded in the browser. One of the key goals of RIAs is to reduce the amount of extra data transmitted with each request. Rather than download an entire page, why not download only the data that changed and update the page the user is viewing? This is the way standard desktop and client/ server applications work. Although this goal seems simple and is readily accepted by developers taking their first plunge into RIA development, web developers often bring a page-based mindset to RIAs and struggle to understand how to face the challenges from the page-based world, such as “maintaining state.” For example, after users log in, how do we know who they are and what they are allowed to do as they navigate around the application? Maintaining state was a challenge introduced by web-based applications. HTTP was designed as a stateless protocol: Each request to the server was an atomic unit that knew nothing about previous requests. This stateless nature allowed for greater efficiency and redundancy
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because a connection did not need to be held open between browser and server. Each new page request lasted only as long as the server spent retrieving and sending the page, allowing a single server to handle a much larger number of simultaneous requests. But the stateless nature of the web added challenges for application developers. Usually, applications need to remember information about the user: login permissions, items added to a shopping cart, and so on. Without the capability to remember this data from one request to the next, true application development would not be possible. To help solve this problem, a series of solutions were implemented revolving around a unique token being sent back to the server with each request (often as cookies, which are small text files containing applicationspecific identifiers for an individual user) and having the server store the user’s information. Unlike traditional web applications, RIAs can bypass many of these problems. Because the application remains in client RAM the entire time it’s used (instead of being loaded and unloaded like a page-based model), variables can be set once and accessed throughout the application’s life cycle. A different approach to handling state is just one of many places in which building applications requires a slightly different mindset than web application development. In reality, web-based RIAs bear more resemblance to client/server applications than they do to web applications.
The Advantages of Rich Internet Applications Unlike the dot-com boom days of the mid- to late 1990s, businesses are no longer investing in Internet technologies simply because they are “cool.” To succeed, a new technology must demonstrate real return on investment and truly add value. RIAs achieve this on several levels: They can reduce development costs and add value throughout an organization.
Business Managers Because users can work more easily with RIA software, the number of successful transactions is increasing. Occurring across many industries, this increase can be quantified: Businesses can measure things like the productivity of employees using intranet applications or the percentage of online shoppers who complete a purchase. More productive employees, making fewer mistakes, can drastically reduce labor costs, and growing online sales increase revenue and decrease opportunities lost to competitors.
IT Organizations Breaking away from page-based architectures reduces the load on web servers and reduces overall network traffic. Rather than entire pages being transmitted over and over again, an
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entire application is downloaded once; then, the only communication back and forth with the server is the data to be presented on the page. Easing the server load and network traffic can noticeably reduce infrastructure costs. RIAs that are developed using sound architectural principles and best practices can also greatly increase the maintainability of an application as well as greatly reduce the development time to build the application.
End Users As the success of devices such as Apple’s iPhone and Google’s Android-based phones clearly demonstrates, a rich user experience matters, and people are willing to pay for it. End user experiences are one of the greatest benefits of RIAs. A well-designed RIA greatly reduces users’ frustration levels because they no longer need to navigate several pages to find what they need, nor do they have to wait for a new page to load before continuing to be productive. Additionally, the time users spend learning how to use an application can be greatly reduced, further empowering them. Today, there are a number of excellent applications that would not be possible without the concepts of an RIA, such as the MLB.TV Media Player at MLB.com, which delivers real-time and on-demand games with statistics and commentary, or the FedEx Custom Critical shipment tracking dashboard that tracks high-value shipments as they move toward delivery. These applications provide excellent examples of the ease of use an RIA can offer an end user.
RIA Technologies Today, developers have several technology choices when they start building RIAs. Among the more popular technologies are those based on HTML, such as AJAX (Asynchronous JavaScript and XML), and those based on plug-ins. Plug-in-based options execute code written in languages such as MXML/ActionScript or XAML/.NET in a virtual machine (Flash Player and Silverlight, respectively); these are installed inside the web browser. The current RIA landscape offers four paths: HTML/AJAX, Java, Microsoft Silverlight, and Adobe Flash Platform. Each has distinct advantages and disadvantages when applied to an RIA. As with many things in life, the key is discovering the right tool for your circumstance.
Asynchronous JavaScript and XML (AJAX) One of the easier technologies to understand (but not necessarily to implement) is AJAX. It is based on tools already familiar to web developers: HTML, DHTML, and JavaScript. The fundamental idea behind AJAX is to use JavaScript to update the page without reloading it.
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A JavaScript program running in the browser can insert new data into the page or change its structure by manipulating the HTML DOM without reloading a new page. Updates may involve new data loaded from the server in the background (using XML or other formats) or in response to user interaction, such as a mouse click or hover. Early web applications used Java applets for remote communication. As browser technologies developed, other means, such as IFrames (floating frames of HTML content that can speak to each other and to servers), replaced the applets. In recent years, XMLHttpRequest was introduced into JavaScript, facilitating data transfers without the need for a new page request, applet, or IFrame. In addition to the benefit of using familiar elements, AJAX requires no external plug-in. It works purely on the browser’s capability to use JavaScript and DHTML. However, the reliance on JavaScript poses one of the new liabilities of AJAX: It fails to work if the user disables JavaScript in the browser. Another issue with AJAX is that it has varying levels of support for DHTML and JavaScript in different browsers on different platforms. When the target audience can be controlled (say, for intranet applications), AJAX can be written to support a single browser on a particular platform. Many businesses today standardize their browsers and operating systems. However, when the audiences are opened up (such as those for extranet and Internet applications), AJAX applications need to be tested, and often modified, to ensure that they run identically in all browsers on all operating systems. Finally, HTML and JavaScript were not created with applications in mind. This means that, as a project grows in scale, the combination of code organization and maintenance can be challenging. AJAX is not likely to go away anytime soon, and each day more high-profile AJAX applications (such as Google Maps) are launched to great acclaim. In fact, for certain classes of applications, usually smaller widgets and components, AJAX is clearly a better solution than any of its competitors due to its small size and relatively independent deployment model. It should be noted that AJAX is not a programming model in and of itself. It is really a collection of JavaScript libraries. Some of these libraries include reusable components designed to make common tasks easier. Because AJAX lacks a centralized vendor, integrating these libraries introduces dependencies on third parties, which assumes a certain amount of risk. Draft proposals around HTML 5 aim to reduce some of the complexity and certainly the disadvantages of working in this environment. However, at this time we must take a wait-and-see approach to identifying how organizations producing browsers adopt and implement this technology.
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Java Virtual Machine Java—and the languages and frameworks built on Java—are the de facto standard for most enterprise server applications. However, Java has had a traditionally difficult time on the client for many reasons, including download size and perceived and real difficulties maintaining and updating the Java runtime. Although there are certainly some success stories of Java client-side code, such as the Eclipse project (the development environment that Flash Builder itself uses), Java is still struggling to gain market share in the RIA space. The current offering from the Java community in the RIA space is called JavaFX, a software platform designed to run across many JavaFX-enabled devices. While JavaFX has been embraced strongly by some of the Java community, it has so far received a lukewarm welcome by those not already committed to Java as their end-to-end solution.
Microsoft Silverlight As of the time of writing, Microsoft has just released a beta version of Silverlight 4.0, its solution for building RIAs. It is a comprehensive offering whose many pieces include: • Silverlight—A web-based application framework that provides application functionality, written in a subset of .NET. • XAML—Extensible Application Markup Language. The XML-based language in which you declare user interfaces, XAML is somewhat analogous to Flex’s MXML language, which you will learn about shortly. • Microsoft Expression Studio—A professional design tool intended to build Windows client applications as well as web applications targeted for the Silverlight player. The suite specifically includes: • Expression Web—A visual HTML editor • Expression Blend—A visual application that builds and edits applications for WPF (Windows Presentation Foundation) and Silverlight applications • Expression Designer—A graphics editor • Expression Encoder—A video encoder With these tools, Microsoft is promoting a workflow in which designers create compelling user interfaces with Expression (using WPF or Silverlight), and then developers implement the business and data access logic.
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As of the time of writing, Windows and Mac OS X support Silverlight. A Linux and FreeBSD implementation written by Novell, named Moonlight, brings this content to those operating systems. Silverlight is beginning to offer a compelling platform, especially for .NET developers who can use tools such as Microsoft Visual Studio and leverage existing knowledge to quickly create new applications. The only limitation that Microsoft seems to have at this moment is the distribution of the Silverlight runtime, which is still limited even on Windows, let alone Mac and Linux. Silverlight is absolutely worth watching over the coming year.
Adobe Flash Platform One of the leading competitors in the RIA space is the Adobe Flash Platform. The platform is composed of a number of tools and technologies designed to provide rich and compelling user experiences while maximizing developer productivity and designer involvement. While many pieces to the Flash Platform can be included as needed for different types of applications, the following sections include the ones most relevant to this book.
Adobe Flash Player and AIR Originally written as a plug-in to run animations, Flash Player has evolved significantly over the years, with each new version adding capabilities while still maintaining a very small footprint. Over the past decade, Flash Player has gained near ubiquity, with some version of it installed in more than 99 percent of all desktops. Since 2002, Macromedia (now part of Adobe) began focusing on Flash as more than an animation tool. With the Flash 6 release, Macromedia began providing more capabilities for building applications. It found that the ubiquity of the player, combined with the power of the scripting language (ActionScript), enabled developers to build full browser-based applications and get around the limitations of HTML. By targeting Flash Player, developers could also break away from browser and platform incompatibilities. One of the many nice features of Flash Player was that content and applications developed for any particular version of Flash Player would usually run on any platform/ browser that supported that version. With very few exceptions, that remains true today. AIR (Adobe Integrated Runtime) continues the tradition of Flash Player, but removes dependency on the browser. AIR allows content written for Flash Player, in addition to HTML and JavaScript, to run as a desktop application, effectively eliminating the need for ActionScript developers to learn additional languages should they wish to deploy their applications as more traditional applications or to nondesktop devices.
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LESSON 1: Understanding Rich Internet Applications
Flash Professional An industry-leading tool for creating engaging experiences, Flash is familiar to most designers and many developers worldwide. It provides a rich environment to create graphic assets as well as animations and motion. Flash Professional can be used alone to create RIAs; however, the environment is intended for designers creating interactive content and may seem unfamiliar to developers who wish to build RIAs using a more traditional development methodology. Originally, this kept many serious developers from successfully building applications on the Flash Platform, leading to the introduction of the Flex framework.
Flex Sensing the need for RIA tools that were more developer-friendly, Macromedia created a language and a compiler that enabled developers to work with other, familiar languages from which the compiler could create applications to run in Flash Player. In 2004, Macromedia released Flex 1.0 (followed by Flex 1.5 in 2005). Adobe continued this cycle, releasing Flex 2.0, Flex 3.0, and Flex 4 in 2006, 2008, and 2010, respectively. Functionally, Flex applications are similar to AJAX applications, in that both are capable of dynamic updates to the user interface and both include the ability to send and load data in the background. Flex goes further by providing a richer set of controls and data visualization components along with the next generation of developer tools and services to enable developers everywhere to build and deploy RIAs on the Flash Platform. Flex builds on several key technologies and introduces several new pieces: • ActionScript 3.0—A powerful object-oriented programming language that advances the capabilities of the Flash Platform. ActionScript 3.0 is designed to create a language ideally suited for building RIAs rapidly. Although earlier versions of ActionScript offered the power and flexibility required for creating engaging online experiences, ActionScript 3.0 advances the language, improving performance and ease of development to facilitate even the most complex applications with large datasets and fully object-oriented, reusable code. • MXML—A declarative XML syntax that allows for the simplified creation and maintenance of user interfaces. MXML provides an easy way to visualize and implement the hierarchical nature of a complex UI without the need to directly write and maintain ActionScript code. MXML is automatically compiled to ActionScript during the process of building a Flex application.
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• Flash Player 10 (FP10)—Building on the success of Flash Player 9, this next generation of Flash Player focuses on improving script execution and driving Flash Player to additional smaller devices. To facilitate this improvement, FP10 introduces optimizations to the ActionScript Virtual Machine (AVM), known as AVM2, a virtual machine that understands how to execute ActionScript 3.0 code regardless of the device displaying the content (for example, a mobile phone, a TV, or a desktop computer). Major improvements in runtime-error reporting, speed, memory footprints, and improved use of hardware acceleration are allowing developers for this platform to target a variety of devices well beyond the desktop/web browser. Unlike applications built with JavaScript, Flash Player is capable of using a just-in-time (JIT) compilation process that enables the code to run faster and consume less memory, like a native application installed on your computer. • Flex SDK—Using the foundation provided by FP10 and ActionScript 3.0, the Flex framework adds an extensive class library to enable developers to easily use the best practices for building successful RIAs. Developers can get access to the Flex framework through Flash Builder or the free Flex SDK, which includes a command-line compiler and debugger, enabling developers to use any editor they prefer and still be able to access the compiler or debugger directly. • Flash Builder 4—Flash Builder is the newly rebranded IDE (integrated development environment) built on the success of Flex Builder 2 and 3. Providing developers with an environment specifically designed for building RIAs, Flash Builder 4 takes the IDE to the next level. Built on top of the industry-standard open source Eclipse project, Flash Builder 4 provides an excellent coding and debugging environment and promotes best practices in coding and application development. Drawing on the benefits of the Eclipse platform, it provides a rich set of extensibility capabilities, so customizations can easily be written to extend the IDE to meet specific developers’ needs or preferences.
Flash Catalyst Flash Catalyst is a new tool focused on designers creating RIAs. Catalyst allows designers to build applications by selecting portions of preexisting artwork created in Adobe Photoshop, Illustrator, or Fireworks and then indicating how that artwork relates to the application (scroll bar, button, and so on). Using this tool, a designer can create pixel-perfect application shells complete with behaviors, transitions, and even data placeholders that are ready for integration and implementation by a developer using Flash Builder 4. The combination of these tools represents a powerful cross-platform designer-developer workflow built on the industry-standard graphics editing capabilities offered by the Adobe Creative Suite.
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LESSON 1: Understanding Rich Internet Applications
Flash Platform Server Technologies Although beyond the scope of this book, the Flash Platform extends further to the server, providing services and functionality that your RIAs can consume. Available servers and services range from workflow engines to data persistence and from messaging to video publishing and delivery. Although you do not need any of these components to build rich experiences and applications, they do provide amazing capabilities that can extend your applications’ reach and use.
What You Have Learned In this lesson, you have: • Explored the evolution of computer applications (pages 4–6) • Explored alternatives to page-based architecture (pages 6–7) • Explored the benefits of RIAs (pages 7–8) • Compared RIA technologies (pages 8–14)
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LESSON 2
What You Will Learn In this lesson, you will: • Create a Flash Builder project • Understand the parts of the Flash Builder workbench: editors, views, and perspectives • Create, save, and run application files • Use some of the features in Flash Builder that make application development faster and easier, such as code hinting and local history • Work in both Source view and Design view • Use various views, such as the Package Explorer
Approximate Time This lesson takes approximately 1 hour and 30 minutes to complete.
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Lesson 2
Getting Started You’re ready to start your adventure of learning Adobe Flex, so the first thing to do is become familiar with the environment in which you will be developing your applications. This environment is Adobe Flash Builder, which is based on the Eclipse platform. The Eclipse platform is an open source integrated development environment (IDE) that can be extended. Flash Builder has extended and customized Eclipse for building Flex applications. In this lesson, you become familiar with Flash Builder by building the main application files of the FlexGrocer application that you will be working on throughout this book. While working on the FlexGrocer application, you will learn about the Flash Builder interface and how to create, run, and save application files. You will also discover some of the many features Flash Builder offers to make application development easier.
Creating a new project in Flash Builder
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LESSON 2: Getting Started
Getting Started with Flex Application Development Before you can build a building, you must lay the foundation. This lesson is the foundation for further Flex development. You will finish this lesson knowing how to manipulate Flash Builder in ways that make the process of Flex development easier and faster. Along the way, you will create the main application file that defines the FlexGrocer application. Part of the study of any new body of knowledge is learning a basic vocabulary, and in this lesson you will learn the basic vocabulary of both Flex development and Flash Builder. You will understand terms such as view, perspective, and editor in relationship to the Flash Builder workbench. Also, you will understand the terms describing the processes that transform the text you enter in Flash Builder into the type of file you can view with your browser using Flash Player.
Creating a Project and an MXML Application In this first exercise, you will create a Flex application. To do so, you must first create a project in Flash Builder. A project is nothing more than a collection of files and folders that help you organize your work. All the files you create for the FlexGrocer application will be in this project. You’ll also see that you have two choices when working with an application file: You can work in either Source view or Design view. In most cases, the view you choose will be a personal preference, but at times some functionality will be available in only one view. Also in this exercise, you will run the Flex application. You’ll discover how the code you write is turned into a SWF file that is viewed in a browser. 1 Start Flash Builder: On Windows choose Start > Programs > Adobe > Adobe Flash Builder; on Mac OS open the Flash Builder application from the Adobe Flash Builder 4 folder in your Applications directory. This is most likely the way you will start Flash Builder. You may already have Eclipse installed on your computer and previously added the Flex functionality using the plug-in configuration. In that case, you need to open Eclipse as you have before, and switch to the Flash perspective. 2 Choose File > New > Flex Project. For the Project name, enter FlexGrocer. Deselect the “Use default location” check box, and for the Folder location enter driveroot:/flex4tfs/ flexgrocer. Be sure that Web is selected as the application type, and that the application server type is set to None/Other.
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Creating a Project and an MXML Application
19
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The project name should reflect the files contained in the project. As you continue your work with Flash Builder, you’ll soon have many projects, and the project names will help remind you which files are in each project. Do not accept the default location entry. The default uses your Documents directory and places files very deep in the directory structure. For simplicity’s sake, you are putting your work files right on the root drive. Flash Builder lets you choose whether to use the most recent compiler (the default choice) or one from a previous version, by selecting the appropriate “Flex SDK version” radio button. For this application, you should use the Flex 4.0 compiler. 3 Click Next.
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LESSON 2: Getting Started
4 Leave the output folder for the compiled Flex application as bin-debug. At this time, there is no need to change this default. Then click Next.
5 Ensure that FlexGrocer.mxml is set as the main application filename. By default, Flash Builder gives the main application file the same name as your project name. Flash Builder automatically creates the main application file for you and includes the basic structure of a Flex application file.
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Creating a Project and an MXML Application
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NoTe: MXML is a case-sensitive language . Be sure to follow the case of the filenames in tags shown in this book .
6 Click Finish and see the project and application file you created. Here you see your first Flex application. Currently the application is displayed in Source view. Later in other lessons, you will also look at this application in Design view. Source view and Design view buttons Package Explorer
Line numbers
Run button
Problems view
Open Perspective button
Editor
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LESSON 2: Getting Started
The default application file contains some basic elements. The first line of code
is an XML document-type declaration. Because MXML is an XML standard language, the document declaration should be included in the code. Starting with the second line of code,
you will see definitions for a Flex main application and their associated namespaces. A minimum height and width for the application are also defined. The tag represents the outside container, the holder of all the content in the Flex application. You can have only one tag per Flex application. Inside the tag are three attribute/value pairs that refer to URLs, such as xmlns:fx=”http://ns.adobe.com/mxml/2009”. These declarations are XML namespaces: They
say where the definitions for the classes you will use in your application are and how you refer to those classes. In Flex 4 applications, it is common to have the three namespaces provided, as you are likely to reference three particular libraries of classes: Flex language tags, represented by the fx namespace; Flex Spark components, represented by the s namespace; and Flex MX components, represented by the mx namespace. The Flex language tags represent elements that are needed for Flex 4 applications, but these elements are not actually classes within the Flex 4 SDK. You will find the tag one of the more frequently encountered language tags. Declarations will be explained in more detail when events are discussed. The Spark classes represent the new components in Flex 4, and the MX classes represent Flex 3 classes you may use in your application. As you will discover over the next several lessons, some classes in Flex 3 have not yet been created for Flex 4. By using the namespaces for both Flex Spark and MX components, you will be able to mix and match these two sets of components. NoTe: In XML nomenclature, the part of the attribute between the colon ( : ) and the equal sign ( = ) is known as the prefix, and the quoted string after the equal sign is known as the Universal Resource Identifier (URI) . So, given xmlns:s=”library://ns.adobe.com/flex/spark”, s is the prefix, and library://ns.adobe.com/flex/spark is a URI .
In a configuration file called flex-config.xml, an association is made between these URIs and an associated manifest file. Each manifest file contains a delineation of all the legal tags that can be used with that particular prefix and of the location of the class to which each tag
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will refer. In a standard installation of Flash Builder on a PC, the manifest files are located in installationdirectory/Adobe/Adobe Flash Builder 4/sdks/4.0.0/frameworks. On a Mac, the manifest files are found at installationdirectory/Adobe Flash Builder 4/sdks/4.0.0/frameworks. The fx namespace is pointing to the mxml-2009-manifest.xml, the s namespace points to the spark-manifest.xml, and the mx namespace points at the mx-manifest.xml NoTe: If you look in the manifest file for the fx namespace (mxml-2009-manifest .xml), you will notice a conspicuous absence of Declarations as one of the listed classes . In fact fx:Declarations is not a reference to a class, so much as it is a compiler directive, instructing the compiler how to associate metadata with the ActionScript class created from the MXML . It is more important to note that the other two manifest files do indeed contain references to all the classes you will make use of when using their namespaces .
Part of the spark-manifest.xml file is shown here.
Finally, a minimum height and width are defined for the application. By specifying these, Flash Player will know whether the browser that the application is running in is large enough to fit the application. If the browser is not large enough, scroll bars need to be added to allow the user to access the rest of the application.
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LESSON 2: Getting Started
Understanding the Flash Builder Workbench Before you do any more work on your application file, you need to become more familiar with the Flash Builder workbench, which is everything you see in Flash Builder. In this exercise you will learn what views, editors, and perspectives mean in the workbench. 1 Close the current editor by clicking the X on the right side of the FlexGrocer.mxml editor tab. All editors have a tab at the top left of the editor area. Whenever you open a file, it is opened in the workbench in what is called an editor. You just closed the editor containing the FlexGrocer.mxml file. You can have many editors open at once in the workbench, and each will contain a file with code in it. 2 Open the editor containing the FlexGrocer.mxml file from the Package Explorer by double-clicking the filename. You can also open the file by right-clicking the filename and choosing Open. 3 Make the editor expand in width and height by double-clicking the editor tab. Sometimes you will want to see as much of your code as possible, especially because Flash Builder does not wrap text. Simply double-clicking the editor tab expands (maximizes) the editor in both width and height, showing as much code as possible. 4 Restore the editor to its previous size by double-clicking the tab again. As you see, you can easily switch between expanded and nonexpanded editors. 5 Click the Design view button in the editor to view the application in Design view. Your application looks more like the way it will look to an end user. The workbench looks radically different in Design view, which allows you to drag user interface controls into the application. You will also be able to set property values in Design view. 6 Return to Source view by clicking the Source view button in the editor. Most frequently, you will be using Source view in this book, but some tasks are better performed in Design view. 7 Close the Package Explorer by clicking the X on the Package Explorer tab. Just like editors, views have tabs at their top left. In Flash Builder, the sections displaying content are called views. 8 Reopen the Package Explorer by choosing Window > Package Explorer.
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Understanding the Flash Builder Workbench
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After you close a view, you can reopen it from the Window menu. There are many views; in fact, if you choose Window > Other Views you’ll see a window listing many of the views.
9 Click the Open Perspective button just above the top right of the editor, and choose the Flash Debug perspective.
A perspective is nothing more than a layout of views that you want to use repeatedly. Flash Builder comes with built-in Flash and Flash Debug perspectives. You can adjust the layout of views in Flash Builder in any way that works best for you, and save it as a perspective by choosing Window > Perspective > Save Perspective As. Once it’s saved, you can switch to that perspective from the Open Perspective menu at any time. 10 Return to the Development perspective. As you can see, it is easy to switch between perspectives. Later in the lesson, you’ll use the Debug perspective and discover its many helpful options.
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LESSON 2: Getting Started
11 If they are not showing, turn on code line numbers by choosing Window > Preferences. In the dialog box, click the disclosure triangles to the right of General and then Editors to expand the menus. Finally, click Text Editors and select the check box for “Show line numbers.”
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Running Your Application
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Running Your Application In the first exercise, you created your project and an application page. Before you had a chance to run the application, the second exercise took you on a tour of the Flash Builder workbench. You will now get back to your application. You will run it, add code to it, and learn the basics of file manipulation. 1 Open the Project menu. Be sure the Build Automatically option has a checkmark in front of it.
When Build Automatically is selected, Flex continuously checks for saved files, compiles them upon saving, and prepares them to run. Even before you run your application, syntax errors are flagged, which does not occur if Build Automatically is not selected. Tip: As your applications grow more complex, you might find that having this setting selected takes too much time, in which case you should deselect the setting . The build will happen only when you run your application or you specifically choose Build Project from the menu . 2 Run your application by clicking the Run button. You will not see anything in the browser window when it opens.
Run FlexGrocer tip
You have now run your first Flex application, and it wasn’t that interesting. In this case, the skeleton application contained no tags to display anything in the browser. But you did see the application run, and you saw the default browser open and display the results, uninteresting as it was.
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LESSON 2: Getting Started
NoTe: What exactly happened when you clicked the Run button? A number of processes occurred . First, the MXML tags in the application file were translated to ActionScript . ActionScript was then used to generate a SWF file, which is the format Flash Player understands . The SWF file was then sent to Flash Player in the browser . www.flexgrocer.com/FlexGrocer.html MXML ActionScript Client with Flash Player
SWF FlexGrocer.swf
3 Close the browser and return to Flash Builder. 4 Add an tag by placing the cursor after the closing tag; press Enter/Return; enter the less-than sign (). Check to be sure that your code appears as follows:
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LESSON 2: Getting Started
NoTe: The code in this example places the minWidth and minHeight attribute/value pairs of the Application tag on a separate indented line . The entire Application tag could have been on one line; whether or not to add line breaks to code is a matter of personal preference . Some developers like the look of placing each attribute/value pair on a separate indented line . Proper XML syntax gives you two ways to terminate a tag. You just used one of them—to place a slash at the end of the tag, which is called a self-closing tag. The other option is to use the slash in front of the tag name, which is completely typed out again, as follows:
You usually use the self-closing tag unless there is a reason to place something inside a tag block. For example, if you want to place the tag inside the tag block, you have to terminate the tag on a separate line. 8 Save the file and run it. The text “My First Flex Application” appears in your browser. Finally, you get to see something displayed in your new Flex application.
The tag comes with a default background color of white (#FFFFFF). You will learn more about adjusting styles in Lesson 16, “Customizing a Flex Application with Styles.” 9 Change the value of the text property from “My First Flex Application” to New Text. Save the file and run it. The next step shows another helpful feature of Flash Builder, but to see this feature you must have at least two saved versions of the file, which is why you changed the text and saved another version of the file. 10 Right-click in the editor, and from the contextual menu choose Replace With > Local History. A large dialog box appears.
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Running Your Application
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11 Compare the current version of your file, which is located on the left side of the dialog box, to the older version on the right. Select the first item in the list. A history of the last 50 versions of your file is kept at the top of the dialog box. Click Replace to bring back your original text, which reads “My First Flex Application”. You will find this feature very helpful when you want to roll back to a previous version of code.
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LESSON 2: Getting Started
13 Run the application. You will see the following warning, telling you there are errors in your project and prompting you to confirm that the launch should continue. In this case, click Cancel.
Because of the compile time error, Flash Builder will not be able to compile the application with the latest change. If you click Proceed in this dialog box, Flash Builder will run the last successfully compiled version of your application. 14 Correct the error, save the file, and run it to be sure that everything is again working properly. Close the file.
Exploring the Flash Builder Debugger As you build applications, things will sometimes not work properly or will perhaps throw errors. To help you understand what is causing these problems, Flash Builder has an interactive debugger. The debugger lets you set breakpoints in the code and inspect various property and variable values at the point where the execution of the code stops. You can also use the debugger to step through the code, so you can see how those values change over time.
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Exploring the Flash Builder Debugger
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1 From the main menu of Flash Builder, choose File > Import > Flash Builder Project. This project file contains a small application with a button, a label, and an ActionScript that will add two numbers and display the results. Flash Builder has the ability to import preexisting projects packaged in the FXP format as a stand-alone file. 2 In the Import Flash Builder Project dialog box that opens, click the first Browse button on the right of the screen. Navigate to the flex4tfs/Lesson02/independent directory, and select the DebugTest.fxp file. Set the location for “Extract new project(s) to” as driveroot:/ flex4tfs/debugTest.
If the debugTest directory does not exist (and it probably doesn’t, unless you have done these steps before), it will be created for you. If it does exist, and you want to replace the previous version, be sure to select the “Overwrite existing project” radio button in the “Import method” section of the dialog box. If you are overwriting an existing version, you will be prompted to confirm that the previous version is to be replaced. At this prompt, you will need to click OK to continue. 3 Click Finish.
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LESSON 2: Getting Started
4 In Flash Builder, notice that a new debugTest project has been created. Expand the src and default package nodes to find the DebugTest.mxml file. Open DebugTest.mxml by double-clicking it.
5 Run the application and click the Click Me button. You should see a line of text appear next to the button, reading “2 + 4 = 6”.
When you clicked the button, the event handler defined on line 26 was executed, calling the button_clickHandler() method. This method defines two integer variables (numTwo and numFour) and passes them to a function that adds the integers together and then displays the results in a label. The ActionScript syntax, event handlers, and datatyping variables will all be covered in detail in the next few chapters. 6 In Source view, double-click the line number 24 to set a breakpoint on that line. You need to double-click the number itself, not merely that line of code. When the breakpoint is set, you should see a blue circle appear to the left of that line number.
When the application runs in Debug view, and a user clicks the button, the application will stop executing at the line with the breakpoint. You will be able to debug from that point in the application. You can set breakpoints at any line of executable code in an application, such as an event handler in MXML, or a line of ActionScript in the Script block.
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Exploring the Flash Builder Debugger
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7 Launch the application in Debug view, by clicking the button that looks like a bug (located to the right of the Run application button).
8 When the browser launches the application, click the button labeled Click Me. This time the application runs until it reaches the breakpoint, then control is passed from the browser to Flash Builder. If Flash Builder is in the Flash perspective, you will be asked if you want to switch to the Debug perspective. Select the Remember My Decision check box, then click Yes.
As you learned earlier in this chapter, Eclipse (and therefore Flash Builder) uses perspectives to group together sets of views. By default, the Debug perspective adds four views above the code. To the left is the Debug view, which shows where the application encountered the breakpoint. This view also has buttons to continue running the application, to stop debugging, and to step into the code, step over a line in the code, or return to the place which called the current function. To the right of the Debug perspective are three tabbed views. The Variables view shows the current state of the variables, the Breakpoints view shows all the breakpoints, and the Expressions view shows any watch expressions you have added. You will explore these views and buttons in the next few steps.
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LESSON 2: Getting Started
Step Into Step Over Terminate Resume
Step Return Variables
Breakpoints
Expressions
9 In the Debug view, click the Step Into button, which will move control to the button_clickHandler() method. The Debug view is showing you that you’re looking at the button_clickHandler() method. The Variables view will still show this event, as it did initially, but three new items are visible there as well, representing the three variables defined locally to the function. Since you have not yet executed the line of code that instantiates and sets the values of those variables, they currently have a value of undefined.
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Exploring the Flash Builder Debugger
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10 Click the Step Over button and notice that control is moved to the next line of executable ActionScript. Control stops immediately before this line executes. Click Step Over again to execute the code on the line that instantiates a variable named numTwo and assigns it a value of 2. Now that the numTwo variable has a value, you can see the value in the Variables view.
11 In the Variables view, click the value numTwo, and change it from 2 to 3. The Variables view lets you change the values of variables and see what effect the change has on the application. Changing this value won’t change the underlying code that set the value, but it will let you see what happens if the value is different.
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LESSON 2: Getting Started
12 In the Debug view, click the Resume button to allow the application to continue executing. This time, the label reads “3 + 4 = 7”, as we changed the value of the numTwo variable from 2 to 3.
13 In the Debug view, click the Terminate button (red square) to stop this debugging session. Double-click the line number 14 to set a breakpoint there. You now have breakpoints set at lines 14 and 24.
14 Click the Breakpoints tab to see that view and notice that you now have breakpoints set on lines 14 and 24. You can turn on or off breakpoints by clicking the check boxes next to each breakpoint. At this point, we no longer want to use the breakpoint at line 24, so deselect its check box. Deselecting the check box leaves the breakpoint in place but instructs Flash Builder to ignore it for now. You will notice that the icon on that line of the code has changed from a
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Exploring the Flash Builder Debugger
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blue circle to a white circle. If you want to completely remove a breakpoint, you can either double-click its line number again, or right-click the breakpoint in the Breakpoints view and choose Remove.
15 Run the application in Debug view again, and click the Resume button when the application starts. Notice that this time, the execution stops at the breakpoint on line 14, and that the numTwo and numFour variables are already populated. 16 Click the Step Into button to step into the addInts() function. Notice that the Debug view shows a click on the button called the button_clickHandler(), which in turn has now called addInts(). Also notice that the Variables view is showing another set of variables instead of numTwo and numFour, which were variables local to the button_clickHandler() method. It is now showing value1 and value2, the arguments to the method. 17 Click the Step Over button. As it did the previous times you used Step Over, the debugger executes the next line, this time populating the sumInts variable with the value 6. 18 Click the Step Return button. Notice that control returns to the button_clickHandler() method, and that the sumInts variable is now properly populated with the value 6, as it was computed in the addInts() method. Congratulations! You know how to use the debugger. This will be extremely useful as you work through the exercises in this book and as you develop real-world Flex applications. There is one more new and interesting feature available with breakpoints in Flash Builder: conditional breakpoints. You can enable conditional breakpoints by right-clicking a
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LESSON 2: Getting Started
breakpoint (either next to the line numbers, or in the Breakpoints view), and choosing Breakpoint Properties. From the Breakpoint Properties view, you can enable or disable breakpoints. You can also set conditions. For example, you can set a breakpoint to fire only if a variable has a certain value, or when a potential breakpoint is encountered a certain number of times. While this feature may not be terribly useful as you first start your explorations in Flex, you will find it invaluable as you build more complex applications.
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Getting Ready for the Next Lessons As you go forward though the rest of the book, you will need certain files, such as graphics, to complete the rest of the application. In the same way you imported a project from an FXP file to learn about debugging, you will also import an FXP file that will be the basis for the application you work on throughout this book.
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Getting Ready for the Next Lessons
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When you import a project into Flash Builder, the IDE makes a determination if you already have a project with the same unique identifier (UUID). If you do, it will allow you to overwrite the existing project with the newly imported one. If not, but you already have a project of the same name as the one you are importing, it will ask you to rename the newly imported project. To avoid any confusion, you are going to completely delete the project you had created previously in this lesson, and then import a nearly identical project, which includes some graphics that will be used throughout the rest of the book. 1 In the Package Explorer, right click the FlexGrocer project, and choose Delete.
2 A dialog box will appear asking if you want to delete only the Flash Builder project, or also the files for the project from the file system. Select the radio button to also delete contents.
The project and related files will be removed from the file system.
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LESSON 2: Getting Started
3 From the main menu of Flash Builder, choose File > Import > Flash Builder Project. 4 In the Import Flash Builder Project dialog box, click the first Browse button on the right of the screen. Navigate to flex4tfs/Lesson02/start and select the FlexGrocer.fxp file. Set the location for “Extract new project to” to your project directory.
5 Click Finish. You are now ready to continue through the rest of the book. If you care to verify that the project imported properly, look in the Project Explorer and confirm that there is now an assets directory in your project.
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What You Have Learned
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What You Have Learned In this lesson, you have: • Created a project to organize your application files (pages 18–23) • Toured the pieces of the Flash Builder workbench (views, editors, and perspectives) used to create application files (pages 24–26) • Run and modified application files while using code hinting and local history to produce the code for those files (pages 27–32) • Learned about debugging an application with the Flash Builder debugger (pages 33–40) • Prepared for the next lessons (pages 41–43)
From the Library of Wow! eBook
LESSON 3
What You Will Learn In this lesson, you will: • Use containers • Lay out an application in Source view • Work with constraint-based layouts • Work with view states • Control view states • Lay out an application in Design view • Refactor code as needed
Approximate Time This lesson takes approximately 1 hour and 30 minutes to complete.
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Lesson 3
Laying Out the Interface Every application needs a user interface, and one of the strengths of Adobe Flash Builder 4 is that it enables developers to lay out their application’s interface with ease. In this lesson, you will learn about containers and layout objects in Flex, what differentiates them, and how to use them when laying out your applications. Finally, you will use view states to make the applications dynamically change to react to users’ actions.
The user interface for the e-commerce application
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LESSON 3: Laying Out the Interface
Learning About Layouts Almost all the positioning of components in Flex is accomplished using containers and layout objects. Working with a kitchen analogy for the moment, you can think of the container as a food processor without a blade. There are different food processors with different features on the market, and you need to choose one that works best for your application. You can think of layout objects as blades that can be inserted into a food processor to slice, dice, chop, and so on. Neither of these two pieces is particularly useful without the other, but when they’re assembled, they become a powerful tool. The same is true of containers and layout objects.
Understanding Containers On a technical level, containers are simply a special type of component that contains and groups other items. These items are collectively referred to as children or, more specifically, as LayoutElements (which is just a broad term for components such as Buttons, Checkboxes, and the like) and GraphicalElements such as squares, circles, and so on. Although containers know how to group and keep these items together, they do not know the position or order of those items on the screen. When you select a container to use, you will do so based on a number of criteria; however, the most fundamental is its ability to be skinned. Skinning is the process of defining the visual appearance of a component. In terms of a container, you can think of the visual appearance as including things such as backgrounds, borders, drop shadows, and so on. Some containers in Flex can be skinned, meaning you can define how they appear on the screen. Other containers exist only to ensure that their children are grouped; they do not have a visual display of their own.
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Learning About Layouts
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Container Types Container
Description
Group
The simplest type of container in Flex 4, a group can be used to contain children, but it does not have any visual appearance of its own .
SkinnableContainer
A SkinnableContainer performs all the same functionality as the group but also has the ability to define its visual appearance on the screen .
BorderContainer
A type of SkinnableContainer that can be used to quickly surround children of a container with a border .
Panel
A type of SkinnableContainer, surrounded by a border, that can have a header and a control area called a control bar .
Application
A type of SkinnableContainer that is used as the root of your Flex application . Like the Panel, it can also have a control bar .
NavigationContent
A special type of SkinnableContainer used with a control called a ViewStack, which you will learn to use later in this book .
There are several more Flex containers, including DataGroup and SkinnableDataContainer, in addition to several specialized containers, such as Form, which will be used in the coming lessons. However, those containers follow a slightly different layout metaphor, so they will be introduced a bit later when their specific use can be explained more clearly.
Understanding Layout Objects .
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LESSON 3: Laying Out the Interface
Combining Containers and Layout Objects Once you have chosen a container and a layout object, you assemble them in MXML to produce the desired effect. Look at the following examples of setting a layout object using the layout property to control the positioning of the buttons.
If you do not specify a layout object, BasicLayout is used, meaning you must specify x and y positions for each button or they will all default to appear at the origin coordinates (0,0).
Scrolling Content You will occasionally find a situation in an application where it is desirable to scroll the contents of a group. In previous versions of Flex, every container had this functionality by default. While extremely convenient for the developer, it also meant that every container was burdened with this extra code even though it was hidden the vast majority of times. In Flex 4, you need to specifically indicate when an area is scrollable. This is accomplished via a special tag named Scroller that wraps your Group tag.
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Learning About Layouts
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Just wrapping the Group in a Scroller will not necessarily make a scroll bar appear. The Scroller will add scroll bars (vertical, horizontal or both) as needed when there is not enough room to display the Group at full size. In the previous example, the height of the Scroller is specifically set to 65 pixels to ensure that a vertical scroll bar appears. If you do not set specific width and heights, then Flex will always try to fit the whole Group on the screen first and will resort to scrolling only if that is not possible.
Decoding MXML Tags Before you begin the exercise in the next section, there is an important concept to learn. It is the difference between class instances and properties in MXML. If you look at the code snippet from the previous section, you will see a Flex button defined in MXML. Right now the label property of that Button is defined as an attribute of the Button’s XML tag:
However, in MXML, you are also allowed to define this same property using child tags. In that case, the code would appear as follows: 3
These two ways of defining the classes will yield identical results on the screen. After you have used Flex for a while, it will become a natural part of your development process to choose the correct syntax in a given situation; however, it can be very confusing when you are new to the language.
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Now, how do you know, regardless of the definition style, which is a property and which is a class? The key to decoding this logic is to watch the case of the first letter after the namespace (after s: in this example). When the first letter is uppercase, such as the B in Button, the code is referring to a new instance of a class. When the first letter is lowercase, such as the l in label, you are setting a property of a class. If you consider a slightly larger example from the previous code: 3
The G in the tag is uppercase, so it refers to an instance of a Flex class named Group. The l in the tag is lowercase, so it is a property of the Group. The V in the tag is uppercase, so it is referring to a new instance of a Flex class named VerticalLayout. If you were to translate the code into words, it would read as follows: Create an instance of the Group class. Set the layout property of that Group instance to a new instance of the VerticalLayout class. Add three Buttons to that Group with the labels 1, 2, and 3, respectively. Make sure this section makes complete sense before continuing in the book. If you ensure you understand these points, the rest of this lesson will be smooth sailing. If you are unsure, the remainder can be a very disheartening experience.
Laying Out the E-Commerce Application The e-commerce application of FlexGrocer is the means through which customers shop for groceries. The top region of the application’s user interface displays the store logo as well as navigation links that appear throughout the application. Below that is a series of clickable icons that users can use to browse the various categories of groceries (dairy, meat, fruit, and so on). Below the icons is an area for displaying products.
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In this lesson, you will use both Design view and Source view to begin laying out the application. Design view is a powerful feature of Flash Builder but can be a very frustrating experience, especially when you are new to it. It is often very difficult to get objects to align correctly or to be placed inside the intended container on the screen. Therefore, you’ll also see a code sample for everything you do in Design view. If your interface does not look like the one in the book as you proceed, feel free to switch to Source view and make the changes before switching back to Design view.
Starting Your Layout in Source View The first steps of laying out your new application will be done in Source view as you define the area of your application that will hold the logo and some navigation elements. 1 Open the FlexGrocer.mxml file that you created in the previous lesson. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson03/start folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Ensure that Flash Builder is in Source view. To switch between Design view and Source view in Flash Builder, click the buttons in the title bar near the top of the window. 3 Find and delete the Label tag with the text “My First Flex Application” that you added in the last lesson. 4 Insert a new controlBarLayout tag pair in place of the Label tag you just removed.
This tag starts with a lowercase letter, indicating that it is a property of the Application object. A control bar is a section of a container that is visually distinctive. In this application, you are going to use the control bar to hold a logo and some buttons for navigation. 5 Immediately inside the controlBarLayout tag pair, place a self-closing tag.
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Remember, a self-closing tag simply means that instead of having an open tag and a close tag like the controlBarLayout, you have just a single tag that ends in a forward slash and a greater than sign (/>). Adding the tag tells the Application that you want to use absolute positioning inside the control bar for this application. In other words, you will take responsibility for positioning the x- and y-coordinates of the items inside it. 6 Directly below the controlBarLayout tag pair, add a new tag pair named . Inside this tag, you will define which items should appear in the control bar. 7 Add a Button tag inside the controlBarContent tag pair and set its label property to Flex Grocer.
Setting the label property of this Button will make it display “Flex Grocer” on the screen. Because you added this Button inside the controlBarContent tag pair, the Button will appear in the control bar area of the application. Ensure that your code looks like the following sample before continuing:
8 After verifying that your code looks like the example, save the FlexGrocer.mxml file and make sure you do not have any errors in the Problems view.
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9 Choose FlexGrocer from the Run menu to execute your application in the web browser.
When your application launches, you should see a gray block near the top of the screen. This is the control bar. Inside that control bar you should see a single button with the words “Flex Grocer”. While the application may not do much yet, you have actually defined several properties, used a layout object, and added a child object to a container. It will get easier and faster from here. When you are finished admiring your work, close the web browser and get ready to learn about Design view.
Continuing Your Layout in Design View .
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3 Next, click the Flex Grocer Button instance that you positioned in the previous exercise. When you click the Button, the Properties panel on the right side of the screen will change to read s:Button, indicating that the Button you just selected is now being configured. 4 Toward the bottom of the Properties view, expand the Size and Position category by clicking the triangle next to the word Size (you may need to scroll down depending on your screen resolution), there are text boxes for Width, Height, X, and Y. Use the X and Y fields to set the x-coordinate to 5 and the y-coordinate to 5. When you change the y-coordinate, the control bar will grow to accommodate the position of the Button. Later in the book, you will apply styles to set the company logo colors and size. For now, you are just placing the Button in the appropriate position. This is an example of using absolute-coordinates positioning. 5 Find the Components view; by default this will be in the lower-left corner of your screen. Open the Controls folder by clicking the triangle next to the word Controls, and drag a Button control to the control bar so the Button control is positioned near the right edge of the control bar. In the Properties view, give the Button control the ID btnCartView and the label View Cart.
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Tip: A blue bar will appear, indicating where other components exist horizontally or vertically from your position . This line will aid you in quickly lining up multiple components Don’t worry about the exact x and y placement. Later in this lesson, you will learn how to use a constraint-based layout to position the button so that its right edge is always 10 pixels from the right edge of the Application object. 6 Drag a second Button control to the control bar, just to the left of the first Button control. In the Properties view, give the new Button control the ID btnCheckout and the label Checkout. FlexGrocer users will click this button to indicate that they are finished shopping and want to complete the purchase of the selected products. Again, the exact placement will happen later in this lesson, when you learn about constraint-based layout. 7 Drag a Label control from the Controls folder and place it near the bottom-right edge of the screen. Double-click the Label and set the text property to (c) 2009, FlexGrocer. Much like the Button controls you just added, you needn’t worry about the exact placement of the Label control because it will be handled later with constraints. 8 In the Components panel, collapse the Controls folder and expand the Layout folder. 9 Drag a Group container from the Layout folder of the Components view and place it in the large white area below the control bar. Use the Properties panel to set the ID of the Group to bodyGroup. Then set both the height and width properties to 100% and the xand y-coordinates to 0. 10 With the bodyGroup still selected, scroll to the bottom of the Properties panel. You will see a Layout drop-down menu. Choose spark.layouts.HorizontalLayout, indicating that you would like this Group to arrange its children horizontally.
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This Group will hold the product details and shopping cart for the application. Remember that a Group with a HorizontalLayout displays its children horizontally. You will have products shown on the left and the shopping cart on the right. 11 Drag another Group container from the Layout folder of the Components view and drop it inside the existing Group that you named bodyGroup. In the Properties view, give this new Group the ID products and then assign a height of 150 and width of 100%. 12 At the bottom of the Properties panel, assign the new Group a spark.layouts.VerticalLayout, indicating that you would like this Group to arrange its children vertically. This vertical group will hold the details for a product. 13 Before continuing, switch to Source view and ensure that your code matches the following code. If any tags are different or missing, fix them before continuing. It is okay if your code has slightly different values for the x and y properties of the Checkout Button, View Cart Button, and Label, as you have not set those yet.
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Defining the Product Section Once you verify that your source code matches the example code, switch back to Design view, where you will continue defining the product section. Now, you will begin defining the controls that will eventually represent all the products in your online grocery store. Tip: Sometimes when switching between Source and Design views, you can lose track of the Flash Builder Properties panel in Design view . If this panel ever goes missing, simply choose Windows > Properties to bring it back . 1 Drag a Label control from the Controls folder of the Components view to the vertical Group with the ID of products that you added in the previous exercise. When looking at Design view, this vertical group will have a faint border starting directly below the control bar and continuing down for 150 pixels, where it crosses the screen. You can drop the Label anywhere in this area. 2 Set the ID of the Label control to prodName and the Text property to Milk. 3 Drag a second Label control below the first one. Give the second one the ID price and set $1.99 as the Text. Because these new Label controls are children of the Group container, and the Group has a VerticalLayout object, the product name appears above the price of the product. Tip: If you open Outline view by clicking the Outline tab (this is adjacent to the Components tab you have been using so far), you can see the hierarchy of your application . The root is the Application tag, which contains a Label (copyright) and a Group named bodyGroup as children along with controlBarContent and a controlBarLayout as properties . You can also see the various children of the controlBarContent and the bodyGroup . If you expand the Group named products, you will see the two labels you just added . This is a useful view if you want to make a change to a component . It can be difficult to select just the products Group in Design view . You can easily select it by clicking it in Outline view . 4 Add a Button control below the two Label controls, with an ID of add and the label AddToCart. For each product, you want the name of the product and its price to be displayed. The AddToCart Button gives users the ability to add a product to their shopping cart. Because the two Label controls and the Button control are in a group with a vertical layout, they appear one above the other. You’ll add functionality for the Button control in a later lesson. 5 Save the file and click Run.
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You can clearly see the difference between elements in the control bar section and those in the body.
Working with Constraint-Based Layouts Flex supports constraint-based layouts that let you arrange elements of the user interface with the freedom and pixel-point accuracy of absolute positioning while being able to set constraints to stretch or move the components when the user resizes the window. This method of controlling the size and position of components is different from laying out nested containers (like the Group containers in the previous exercise). In constraint-based layouts, all the controls are positioned in relation to the edges of a parent container, which has been set with a BasicLayout to allow absolute positioning. With the exception of some specialized containers such as Form (which you will use in subsequent lessons), you can use the BasicLayout on any Group or SkinnableContainer, including Application and Panel. Containers using a BasicLayout object require that elements be positioned to absolute coordinates; however, layout constraints allow users to dynamically adjust the layout based on the window size of their browsers. For example, if you want a label to always appear in the
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bottom-right corner of an application regardless of the browser size, you can anchor the control to the right edge of the parent container. The control’s position is then always maintained relative to the right edge of the parent container. In Flex, this is accomplished via layout anchors. They are used to specify how a control should appear relative to the edge of the parent container. To ensure that a control is a certain distance from the bottom and right edges, you will select the check boxes below and to the right of the control in the Constraints area in the Layout section of the Properties view, and use the text boxes to specify the number of pixels away from the edge of the container where you want the control constrained. Flex allows constraints from the top, vertical center, bottom, left, horizontal center, or right of a container. Tip: All constraints are set relative to the edges of the container, as long as the container uses absolute positioning (BasicLayout) . Constraints cannot be set relative to other controls or containers .
1 Open the FlexGrocer.mxml file that you used in the previous exercise. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer-PreConstraints.fxp project from the Lesson03/ intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Find and select the Checkout Button. Toward the bottom of the Properties view, in the Constraints area of the Size and Position section, add a constraint so the right edge of the Button is 10 pixels away from the right edge of the container. Make sure that the Y position of this control is set to 10 pixels.
Y position
Right-edge constraint
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To set a constraint from the right edge, click the rightmost check box above the button icon in the Constraints area. In the text box that appears, enter the number of pixels away from the edge you want the button to be. If the label seems to disappear from the screen, check the scroll bars on the bottom of Design view. By default, Design view shows you just a portion of the application and you may need to scroll occasionally to find what you need. 3 Find and select the View Cart Button. Add a constraint so that the right edge of the button is 90 pixels from the right edge of the container. Make sure that the Y position of this control is set to 10 pixels. You now have it set so that, regardless of the width of the browser, the two navigation buttons are always anchored relative to the top-right edge of the container. 4 Find and select the Label control with the copyright notice. Constrain this Label so that it is 10 pixels above the bottom and 10 pixels away from the right edge of its container. Click the check box in the top-right corner of the Constraints area, and enter 10 in the text box that appears. Also, click the bottom check box and enter 10 in the text box that appears. Because the copyright label is below other containers, it is probably easiest to select it using the Outline view. These settings ensure that, regardless of the width of the Label control, its bottom-right edge will always be 10 pixels above and 10 pixels to the left of the bottom-right corner of the application. If you switch to Source view, the entire file should look similar to the following:
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Your code may differ slightly, depending on the order you added the items and set properties. Don’t worry; the order is not particularly important in this case. Every container and control that you added in Design view is represented by a tag in Source view. When you add elements inside a container, they appear as child tags to the container’s tag. Also note that the layout constraints are set as attributes of the related component. 5 Switch back to Design view and insert a second Group container in the bodyGroup container (the bodyGroup is the first container you added whose width and height are set to 100%). Set the ID of the new Group to cartGroup, clear the Width property so it is blank, and set the Height to 100%. Remember, you can always choose the bodyGroup from the Outline view if you have difficulty finding it. If you accidentally place the new Group in the wrong container, the easiest fix is to switch to Source view and move the tags yourself. The code in Source view for this area should look like this:
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6 In Design view, set the layout of the cartGroup container to VerticalLayout. If you can’t find the cartGroup, remember to choose it from the Outline view and scroll in the Design view window until you see the highlighted container. 7 Add a Label control in the cartGroup container with the text property set to Your Cart Total: $. To the right of the products, there will always be a summary of the shopping cart, indicating whether there are items in the cart and what the current subtotal is. NoTe: At this point you have set the products container to take 100% of the space, but then you added the cartGroup to its right and added a Label . Isn’t that a problem as you are now using more than 100%? Flex enables you to assign more than 100 percent total width or height for a container . Flex containers take this into account and divide the space proportionally based on the requested percentages . Because more space was requested than is available, each request receives a relative portion based on the available space . If any elements were assigned a fixed width (that is, a number of pixels instead of a percentage), the fixed size requests are subtracted from the available space before any relative size requests are allocated .
8 From the Controls folder of the Components view, drag a Button control below the newest Label control and set the label property of the Button to View Cart. This Button will be used to show users the full contents of their shopping cart. If you accidentally place any of the components in the wrong container, the easiest fix is to switch to Source view and move the tags yourself. The code in Source view for this area should look like this:
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9 In the Outline view, choose the Application. In the Properties panel, remove the Min width and Min height values of 955 and 600. As the application runs you can resize the browser and see that the buttons and bottom text are always properly constrained. A minimum width and height would prevent this from occurring on smaller screens. 10 Save the file and click Run.
Working with View States You can use Flash Builder to create applications that change their appearance based on the task the user is performing. For example, the e-commerce application starts by showing users the various products they can buy. When they start adding items to the cart, you want to add something to the view, such as the total cost, so users can get a feel for what is currently in the cart. Finally, users need a way to view and manage the full contents of the shopping cart.
Creating View States In Flex, you can add this kind of interactivity with view states. A view state is one of several views that you define for an application or a custom component. Every MXML page has at least one state, referred to as the default state, which is represented by the default layout of the file.
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Additional states are represented in the MXML as modified versions of the base view state or of other states. 1 Open the FlexGrocer.mxml file that you used in the previous exercise. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer-PreStates.fxp project from the Lesson03/ intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Ensure you are in design view. If it is not already open, open the States view in Flash Builder 4. If you don’t currently see the States view when you look at Flash Builder in Design view, you can add it to the view by choosing Window > States. Notice that there is already one state created to represent the default layout of the application.
3 Create a new state named cartView, which is a duplicate of . You can create a state by clicking the New State icon at the top of the States view or by right-clicking in the view and selecting the New… option. The cartView state will show users the details of all the items they have added to their cart. 4 With the new cartView state selected, click the products container and set its height and width to 0, then choose the cartGroup container and set its height and width values to 100%. For the cartView, the shopping cart will entirely replace the products in the center of the screen. Therefore, you will resize the products container to take up no space and resize the cartGroup container to take up all the available space. At this point, the controls on your screen will be quite a mess. You will likely see an ugly combination of all the controls in the system on top of each other. This is a very important lesson. In Flex, the width and height properties are used to compute the location of items on the screen. In this case, you told Flex that the products container will not take
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any space, so Flex responded by moving the cartGroup container left to take the newly available space. However, just because an item is not allocated space on the screen does not mean it is invisible. 5 Select the products container and change its visible property to false. You can do this by clicking the CategoryView of the Properties panel, finding the visible property, and changing its value to false. CategoryView
Tip: It has been said several times so far in this lesson, but it is so important it is worth repeating: Use the Outline view to find containers when they are difficult to locate on the screen . 6 Ensure that the cartView state is still selected in States view and then drag a DataGrid control from the Controls folder of the Components view and drop it below the View Cart button. Set the ID of the DataGrid control to dgCart, and set the DataGrid control’s width to 100%. In a later lesson, the DataGrid control will be used to show the user the full contents of the cart.
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Make sure you are adding the DataGrid control to the cartGroup container. Your application and code will look a bit different if you accidentally add the DataGrid control before the cartGroup container. If you look at the file in Source view, you should see that the DataGrid has been added to the following code:
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7 Save the file. Note some of the new syntax added during this operation. First, in the DataGrid class, you will see the includeIn property, indicating that this control should appear on the screen only when in the cartView state. Second, the products container still has a width of 100% and height of 150; however, it also has width.cartView=”0” and height.cartView=”0”. This syntax instructs Flex to set those properties in the corresponding states. Testing the file now shouldn’t show any differences because you haven’t added any ability for the user to toggle between the states. In the next exercise, you will add that navigational ability.
Controlling View States Each MXML component has a property called currentState. You can use this property to control which state of the application is shown to a user at any given time. 1 Open the FlexGrocer.mxml file that you used in the previous exercise. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer-PreControl.fxp project from the Lesson03/ intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Switch to Design view and, if it is not already open, open the States view in Flash Builder and select State1 to set the current state. You will add functionality to the default view state so that users can navigate to the other states of the application. 3 Choose the View Cart Button control from the cartGroup container. In the Properties view, set its On click: property to this.currentState=’cartView’.
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Events such as the Button’s click will be explored in detail in Lesson 5, “Handling Events.” The important thing to understand now is that when the user clicks the link, the view will change to the cartView state. cauTioN! The state name is case sensitive and must exactly match the name as you typed it in the previous exercise . You must use single quotes around the state name when entering it in Design view .
4 Choose the View Cart Button control from the control bar. In the properties view, also set its On click: property to this.currentState=’cartView’. You now have two ways to enter the cartView state. 5 Switch to the cartView state. Add a new Button control below the DataGrid control with the label set to Continue Shopping and the click property set to this.currentState=’’. Setting currentState to an empty string resets the application to its default state. 6 Delete the View Cart Button that is inside the cartGroup from the cartView state. When the user is viewing the cart, there is no need for a View Cart Button. You can delete the Button by selecting it in Design view and pressing Delete. The completed application as shown in Source view should read as follows:
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7 Save and test the application. You can now navigate between the states by clicking the buttons to which you added code.
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Refactoring Refactoring is one of the least understood and most useful tools in a developer’s arsenal. It is particularly relevant for a Flex and ActionScript developer, because dynamic interfaces are often recombined with code during the prototype and development stages of a project. Refactoring is simply the process of reorganizing your code in a way that is better suited to a long-term goal without changing the way it functions. Long-term goals might include increasing the maintainability of the software, modifying the architecture to make additional development steps easier, or simply changing the location and structure of the project to make it more understandable to a new developer. However, one thing is always true: At the end of a successful refactoring session, the changes will be imperceptible to an individual running the application who does not look at the source code. The application functions the same way. Many developers find this notion and this process frustrating. Why would you want to spend time changing code you have already written if it makes no noticeable change in the application’s execution? The answers are varied, but here are a few important ones. • Learning: Learning a new language and continuing to use it is a learning experience. You will be learning new things and techniques every day. That often leads to the realization that the code you wrote days, weeks, or months ago may be inefficient, or even ineffective in certain circumstances. Keeping a keen eye on what you have written in the past and being willing to revisit it often provides a more cohesive code base and tighter, more maintainable code. • Duplication and redundancy: As you are developing, it is extremely common to need the same functionality in multiple places in your application. Usually due to time constraints, this code stays forever duplicated. One of the many problems with this is that later, when that code needs to change, you have to be sure to hunt down and fix all the places it is used. A willingness to avoid duplication and move shared code into new places as you continue developing can not only eliminate large headaches down the road, but can also make your day-to-day development more efficient and faster. • The big picture: Often it is difficult, if not impossible, to know how all the pieces in a system will fit together when you begin writing code. If these pieces written early in the process are set in stone, you will end up bending or breaking code down the line when you try to integrate. If you are comfortable with the idea of refactoring your code as needed throughout the process, you can hone your vision of the system as you progress, ending up with objects and structures that work more cohesively when they’re finished.
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We have a couple of reasons for addressing refactoring here. First, many new developers to the Flex and ActionScript world attempt to apply a rigid approach to their development that does not include refactoring. Over the course of time, we have noticed that these developers, above all others, struggle against the language instead of learning to use it as a tool. We simply want you to avoid that pain. Second, throughout this book you are going to be learning. In fact, quite often you are going to learn multiple techniques for accomplishing the same goal. It is not always feasible to introduce the one “right” way to do something from the beginning because these concepts tend to build upon each other. So, once you have learned enough to approach writing something in a new way, you will end up refactoring it. This provides two benefits: the ability to understand multiple ways to accomplish a goal (and hopefully the reasons why you would or would not use one or the other) and the ability to hone the code base into a final application with valid examples for reference. That said, you are going to refactor the application you have built to date, to make it easier to maintain as you continue through this book.
Using Composed Containers As you learned in this lesson, most containers in Flex accept a layout object that dictates the orientation of their children. This layout object is generally specified by adding a LayoutObject to a Group using the layout property, as the following example shows:
While this provides the utmost flexibility, it does require a little extra typing each time you create a new Group. In a small application this is not a very big issue; however, in a large application, adding layout objects to tens of thousands of Groups can become tedious. To solve this problem, Flex allows you to create new components composed of existing components and properties. You can then use these new constructs as shortcuts to desired functionality. The Flex framework has prebuilt a few of these shortcuts in the form of two containers called VGroup and HGroup. In the following chart, the horizontal columns are functionally equivalent.
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Alternative Shortcuts
Composed Version
If you were to examine the VGroup and HGroup source code, you would find that they are little more than the Group you have already learned to use with the layout property preset for your use. In Lessons 9, “Creating Components with MXML,” and Lesson 18, “Creating Custom Components,” you will learn to create your own components wherever you see a similar opportunity to reuse code.
Refactoring Your Application In this section, you will convert all the Groups with HorizontalLayouts to HGroups, and Groups with VerticalLayouts to VGroups. The goal of this exercise is to successfully change the internal structure of the application without changing its functionality. 1 Open the FlexGrocer.mxml file that you used in the previous exercise. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer-PreRefactor.fxp project from the Lesson03/ intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve.
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2 Switch to Source view. 3 Find the group named bodyGroup and change it to an HGroup. Be sure to also change the closing tag for this group. 4 Eliminate the tag for the layout property and the HorizontalLayout object from within the bodyGroup. 5 Find the products group and change it to a VGroup. Be sure to change the closing tag as well. 6 Eliminate the tag for the layout property and the VerticalLayout object from within the products group. 7 Repeat this process for the cartGroup. When you are finished refactoring the application, your code should appear as follows:
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8 Save the file and click Run. You should have the same functionality with the View Cart Button as before and see absolutely no change in functionality, yet have slightly more maintainable code.
What You Have Learned In this lesson, you have: • Used containers and layout objects (pages 46–50) • Begun an application layout in Source view (pages 50–53) • Laid out an application in Design view (pages 53–58) • Worked with constraint-based layouts (pages 58–63) • Worked with view states (pages 63–67) • Controlled view states (pages 67–70) • Refactored your application (pages 70–74)
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From the Library of Wow! eBook
LESSON 4
What You Will Learn In this lesson, you will: • Define the user interface (UI) for the e-commerce FlexGrocer application • Use simple controls such as the Image control, text controls, and CheckBox control • Define the UI for the checkout screens • Use the Form container to lay out simple controls • Use data binding to connect controls to a data model
Approximate Time This lesson takes approximately 45 minutes to complete.
From the Library of Wow! eBook
Lesson 4
Using Simple Controls In this lesson, you will add user interface elements to enable the customer to find more details about the grocery items and begin the checkout process. An important part of any application is the user interface, and Adobe Flex contains elements such as buttons, text fields, and radio buttons that make building interfaces easier. Simple controls can display text and images and also gather information from users. You can tie simple controls to an underlying data structure, and they will reflect changes in that data structure in real time through data binding. You are ready to start learning about the APIs (application programming interfaces) of specific controls, which are available in both MXML and ActionScript. The APIs are fully documented in the ActionScript Language Reference, often referred to as ASDoc, which is available at http://help.adobe.com/en_US/AS3LCR/Flex_4.0/. The Flex framework has many tools that make laying out simple controls easier. All controls are placed within containers (see Lesson 3, “Laying Out the Interface”). In this lesson, you will become familiar with simple controls by building the basic user interface of the application that you will develop throughout this book. You will also learn about timesaving functionality built into the framework, such as data binding and capabilities of the Form layout container.
FlexGrocer with Image and Text controls bound to a data structure
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Introducing Simple Controls Simple controls are provided as part of the Flex framework and help make rich Internet application development easy. Using controls, you can define the look and feel of your buttons, text, combo boxes, and much more. Later in this book, you’ll learn how to customize controls to create your own unique look and feel. Controls provide a standards-based methodology that makes learning how to use them easy. Controls are the foundation of any RIA. The Flex SDK includes an extensive class library for both simple and complex controls. All these classes can be instantiated via an MXML tag or as a standard ActionScript class, and their APIs are accessible in both MXML and ActionScript. The class hierarchy comprises nonvisual classes as well, such as those that define the new event model, and it includes the display attributes that all simple controls share. You place the visual components of your Flex application inside containers, which establish the size and positioning of text, controls, images, and other media elements (you learned about containers in the previous lesson). All simple controls have events that can be used to respond to user actions, such as clicking a button, or system events, such as another component being drawn (events will be covered in detail in the next lesson). You will also learn in later lessons how to build your own events. Fundamentally, events are used to build easily maintainable applications that reduce the risk that a change to one portion of the application will force a change in another. This is often referred to as building a “loosely coupled” application. Most applications need to display some sort of text, whether it be static or dynamically driven from an outside source like an XML file or a database. Flex has a number of text controls that can be used to display editable or noneditable text: • Label: You have already used the Label control to display text. The Label control cannot be edited by an end user; if you need that functionality, you can use a TextInput control. • TextInput: The TextInput control is used for data input. It is limited to a single line of text. • RichText: The RichText control is used to display multiple lines of text, but it is not editable and does not display scroll bars if the usable space is exceeded. • TextArea: The TextArea component is useful for displaying multiple lines of text, either editable or noneditable, with scroll bars if the available text exceeds the screen space available. All text controls support HTML 1.0 and a variety of text and font styles.
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Displaying Images
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NoTe: All four of the text controls mentioned here support Adobe’s Text Layout Framework (TLF) . While you will not be using TLF as part of the application in this book, many new and interesting features are available with TLF . You can learn about TLF on Adobe’s open source site: http://opensource .adobe .com/wiki/display/tlf/Text+Layout+Framework .
To populate text fields at runtime, you must assign an ID to the control. Once you have done that, you can access the control’s properties; for example, all the text controls previously mentioned have a text property. This property enables you to populate the control with plain text using either an ActionScript function or inline data binding. The following code demonstrates assigning an ID to the label, which enables you to reference the Label control in ActionScript:
You can populate any text control at runtime using data binding, which is denoted by curly bracket syntax in MXML. The following code will cause the yourLabel control to display the same text as the myLabel control in the previous example:
Also, you can use data binding to bind a simple control to underlying data structures. For example, if you have XML data, which might come from a server-side dataset, you can use data binding to connect a simple control to the data structure. When the underlying data changes, the controls are automatically updated to reflect the new data. This provides a powerful tool for the application developer. The Flex framework also provides a powerful container for building the forms that we will cover in this lesson. The Form container allows developers to create efficient, good-looking forms with minimal effort. Flex handles the heading, spacing, and arrangement of form items automatically.
Displaying Images In this exercise you will display images of grocery products. To do this, you must use the Image control to load images dynamically. The Image control has the capability to load JPG, GIF, SWF, and PNG files at runtime. If you are developing an offline application that will not access the Internet, you can use the @Embed directive to include the Image control in the completed SWF file.
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1 Open the FlexGrocer.mxml file that you created in the previous lesson. If you didn’t complete the previous lesson, you can import the Lesson04/start files. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Switch Flash Builder to Design view by clicking the Design View button.
3 Be sure that the Components view is open. If it’s not, choose Window > Components.
4 Select the Image control from the Controls folder and drag the control between the Milk and 1.99 Label controls you already added.
When you drag the Image control from the Components view to the container, Flash Builder automatically adds the MXML to place the Image control on the screen and positions it where you drop it.
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Displaying Images
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5 Be sure that the Flex Properties view is open. If it’s not, choose Window > Properties.
The Flex Properties view shows important attributes of the selected component—in this case, the Image control. You can see the Source property, which specifies the path to the Image file. The ID of the Image control references the instance created from the tag or Image class in ActionScript. 6 Click the Source folder icon and navigate to the assets directory. Select the dairy_milk.jpg image and click Open. The image you selected is displayed in Design view. The source property is also added to the MXML tag. 7 Click the Scale content drop-down menu and change the value to true. In an ideal world, all the images that you use in the application would be a perfect size, but this is not always the case. Flex has the capability to set the width and height of images and can scale the image to fit the size of the Image control.
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8 Switch back to Source view and notice that Flash Builder has added an tag as well as the attributes you specified in the Flex Properties window. As you can see, it is easy to switch between Source view and Design view, and each one has its own advantages. Notice as you switch back to Source view that the Image tag you were working on is now highlighted.
9 In the tag that you added, add an @Embed directive to the Image control:
The @Embed directive causes the compiler to transcode and include the JPG in the SWF file at compile time. This technique has a couple of advantages over the default of loading the image at runtime. First, the image is loaded at the start of the application, so the user doesn’t have to wait for the image to load before displaying when it is needed. Also, this technique can be useful if you are building offline applications that do not need to access the Internet because the appropriate images are included in the SWF file and will be correctly displayed when needed. Remember, though, that using this technique greatly increases the size of your SWF file. 10 Save, compile, and run the application.
From the Library of Wow! eBook
Building a Detail View
83
You should see that the Image and Label controls and button fit neatly into the layout container.
Building a Detail View In this exercise, you will use a rollover event to display a detailed state of the application. You will explore different simple controls to display text and review how application states work. 1 Be sure that you are still in Source view in Flash Builder. Near the top of the file, locate the block, which contains definitions for the State1 and cartView states. Add a new state definition named expanded.
You will define this third state for the application to show details of a product. 2 Switch to Design view, set the state selector to expanded, and drag a VGroup from the Layout folder of the Components view into the application. (To position this correctly, you should drag the VGroup below the existing white area.) In the Properties view, verify that the In state’s value is expanded, the X value is 200, and the Width value is 100 percent. Leave the Y and Height values blank.
This new VGroup needs to be positioned as a child of the main application. Since the application has a bodyGroup which is occupying 100% of the space below the control bar, you will need to manually position this in Source view. To do this, switch to Source view, and move the
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to just above the closing tag, so the end of the file reads like this:
3 Switch back to Design view. Ensure that the expanded state is selected in the States view and drag an instance of the RichText control from the Controls folder of the Components view into the new VGroup you created in the previous step.
The RichText control enables you to display multiple lines of text, which you will need when you display the product description that will ultimately come from an XML file. You will use data binding in the next section to make this RichText control functional. For now, you are just setting up the layout. 4 Drag an instance of the Label control from the Components view to the bottom part of the VGroup container you created. Populate the text property with the words Certified Organic. Later on, you will modify the visible property of this component so the contents of the property are displayed only when a grocery item is certified organic.
text
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Building a Detail View
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5 Drag another instance of the Label control from the Components view to the bottom part of the VGroup container you created. Populate the text property with the words Low Fat.
Later, you will set the visible property of this label to true if the grocery item is low fat, or false if it is not. 6 Switch back to Source view. Notice that Flash Builder has added the RichText and two Label controls you added in Design view. Note that all the code created in Design view is displayed in Source view. 7 Locate the tag in the expanded state and set the width property to 50% .
8 Find the tag that is displaying the milk image. Add a mouseOver event to the tag that will change the currentState to expanded. Remove the includeIn attribute.
simply means that when the user rolls the mouse anywhere over the dairy_milk.jpg Image tag, the ActionScript will execute. In this ActionScript, you are referring to the expanded state, which you created earlier in this lesson.
mouseOver
If you had left the includeIn attribute in the image tag, the milk image would appear only in the initial state of State1. Therefore, when you mouse over the image and switch it to the expanded state, the milk bottle image will disappear. By removing the includeIn attribute, you are instructing the application to allow this image to be used in all states. 9 In the same tag, add a mouseOut event that will change the currentState back to the initial state State1.
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When the user moves the mouse away from the dairy_milk.jpg image, the detailed state no longer displays, and by default the application displays only the images and labels for the control, which is expressed with an empty string. 10 Save and run the application. When you roll the cursor over the milk bottle image, you see the RichText and Label controls you created in the expanded state.
Using Data Binding to Link a Data Structure to a Simple Control Data binding enables you to connect controls, such as the text controls that you have already worked with, to an underlying data structure. Data binding is incredibly powerful because if the underlying data changes, the control reflects the changes. For example, suppose you create a text control that displays the latest sports scores; also suppose it is connected to a data structure in Flex. When a score changes in that data structure, the control that the end user views reflects the change. In this exercise, you will connect a basic data structure in an tag to simple UI controls to display the name, image, and price for each grocery item. Later in the book, you will learn more about data models, the effective use of a model-view-controller architecture on the client, and how to connect these data structures with server-side data. 1 Be sure that FlexGrocer.mxml is open, and add an tag after the comment in the tag pair at the top of the page. The tag allows you to build a client-side data model. This tag converts an XML data structure into a format that Flex can use. 2 Directly below the opening tag and before the closing tag, add the following XML data structure. Your tag should look as shown: Dairy Milk
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assets/dairy_milk.jpg 1.20 1.99 true true Direct from California where cows are happiest!
You have defined a very simple data structure inline inside an tag. 3 Assign the tag an ID of groceryInventory. The first line of your tag should look as shown:
By assigning an ID to the tag, you can reference the data with dot syntax. For example, to access the list price of the item, you could use groceryInventory.listPrice. In this case, that would resolve to 1.99. 4 Switch Flash Builder to Design view. You can set up bindings between elements just as easily in Design view as you can in Source view. 5 Select the RichText control in the expanded state and be sure that the Flex Properties view is open. Modify the text property to {groceryInventory.description}. Data binding is indicated by the curly brackets {}. Whenever the curly brackets are used, you use ActionScript instead of simple strings. Effective use of data binding will become increasingly important as you begin to work with server-side data.
6 Save and run the application. You should see the description you entered in the data model when you roll the cursor over the grocery item.
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Using a Form Layout Container to Lay Out Simple Controls Forms are important in most applications that collect information from users. You will be using the Form container to enable the shopper to check out their products from the grocery store. The Form container in Flex will handle the layout of the controls in this form, automating much of the routine work. With a Form container, you can designate fields as required or optional, handle error messages, and perform data checking and validation to be sure the administrator follows designated guidelines. A Form container uses three tags: an tag, an tag, and an tag for each item on the form. To start, the checkout form will be built into a separate application, but later in the book, it will be moved into the main application as a custom component. Form Form Heading
Form Items
Form Item Label
1 Create a new MXML application in your current project by choosing File > New > MXML Application. Name the application Checkout, and choose spark.layouts.BasicLayout as the Layout for the new application. Then click Finish.
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Using a Form Layout Container to Lay Out Simple Controls
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2 Switch to Design view, and drag a Form from the Layout folder of the Components view to the top left of the window. A dialog box will appear asking for the Width and Height of the form. Leave the default values as they are.
3 Drag a FormHeading component from the Layout folder in the Components view into the newly created form. Double-click the FormHeading, and change it to Customer Information.
A FormHeading is a specialized label. Its left edge always aligns with the left side of the form controls (not the labels of the form items.) 4 Drag a TextInput control from the Controls folder of the Components view and drop it just below the FormHeading. The TextInput and a label to the right of the TextInput both appear. Double-click the label, and change it to Customer Name.
When adding controls to a form in Design view, Flash Builder automatically surrounds the control in a FormItem, which is why a label is appearing to the left of the control. If you switch to Source view, you can see the FormItem surrounding the TextInput. Back in Design view, notice how the left edge of the text input is aligned with the left edge of the FormHeading. As noted earlier, this is a feature of the Form and FormHeading classes, and it allows these items to always maintain the left alignment, regardless of the size of the FormItem labels.
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LESSON 4: Using Simple Controls
5 Drag four more TextInputs to the form from the Components view. Change the labels of these to Address, City, State, and Zip. Drag a DateField below all the TextInputs, and change its label to Delivery Date. Drag a button below the DateField, and set its label to be an empty string (simply remove the default text). Double-click the button and change the button’s text to Continue.
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What You Have Learned In this lesson, you have: • Learned how to load images at runtime with the Image control (pages 79–83) • Learned how to display blocks of text (pages 83–86) • Learned how to link simple controls to an underlying data structure with data binding (pages 86–87) • Learned how to build user forms with a minimum of effort using the Form container (pages 88–90)
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From the Library of Wow! eBook
LESSON 5
What You Will Learn In this lesson, you will: • Learn about Flex’s event-based programming model • Pass data to an event handler using MXML • Learn a bit about UI object creation order • Handle both user and system events with ActionScript functions • Understand the event object and its properties
Approximate Time This lesson takes approximately 1 hour to complete.
From the Library of Wow! eBook
Lesson 5
Handling Events An important part of building a rich Internet application (RIA) is building an effective clientside architecture. When you use Flash Builder as an authoring tool, you have the ability to follow object-oriented best practices and an event-based programming model that allows for loosely coupled applications. This type of development is very different for web application developers, because it does not follow a page-based, flow-driven model. Ultimately, using this client-side, event-based architecture can result in better-performing applications that contain more reusable code and consume less network traffic because page refreshes are no longer needed. During this lesson, you will become familiar with an event-based programming model and learn how events are used throughout Flex.
Events are added to the FlexGrocer application to allow the user to interact with the application.
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Understanding Event Handling Flex uses an event-based, or event-driven, programming model: Events determine the flow of the application. For example, a user clicking the mouse button or a server returning data determines what should happen next. These events come in two types: user events and system events. User events are just what you’d most likely guess—a user clicking a mouse or pressing a key. System events include the application being instantiated, an invisible component becoming visible, and many others. The Flex developer handles these events by writing code for what happens next. Tip: Many server-side developers are accustomed to a flow-driven programming model, in which the developer determines the flow of the application rather than having to react to events generated by the user or system; recognizing and understanding that difference is crucial to success with Flex . For the moment, we are going to personify Flex’s event-based model to make its operation clear. Pretend that you and a friend are standing on opposite sides of a parking lot. Your friend is going to act as an event dispatcher, an object that notifies others when something occurs. While you are in the parking lot, your friend may shout a variety of things. He may exclaim, “Car arriving!” “Car departing!” or “Car parking!” Perhaps he periodically decides to simply yell, “Nothing has changed!” In all cases, he is shouting the information, and anyone close can hear it. He has no real control over who hears it, and certainly no control over what a person who overhears these messages might do in response. His only job in this scenario is to announce information, which is precisely the job of an event dispatcher. Now, on the other side of the parking lot, you hear these various messages being announced. You may choose to react to all, some, or none of them. When you hear that a car is parking, for example, you may wish to go and greet the new arrival. However, you may blatantly ignore your friend announcing that nothing has changed or that a car is departing. In this case you are an event listener. While you can hear all the information being broadcast, you decide which messages are important and how you react to them. Just as hearing something is different from actually listening to it in the real world, the same concept exists in event-driven programming. Code that listens to and reacts to a given event is called an event listener or an event handler.
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Understanding Event Handling
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Now, as a last step in our example, imagine that another individual arrives at the parking lot. He can also hear your friend shouting. He may choose to listen and react to the same messages as you, or different ones altogether. Perhaps when he hears that a car is departing, he walks up to the car and asks for payment for parking, while ignoring the other messages. This is the wonderful part about event-based programming. Many people can hear a message, and each can decide whether to react to it. If they do choose to react, they can do so in different ways. Finally, the person doing the shouting doesn’t need to know what might happen as a result; his only job is to keep shouting. Bringing this back to Flex, we may say that an object such as a Button instance dispatches a click event. What we mean is that the Button shouts for all to hear that it has just been clicked. Every other object in the system can choose to listen to that event and to handle it (react). In Flex, if we don’t choose to listen to an event on an object, then we implicitly ignore it. Keeping that in mind, the following general steps occur when a user event occurs and a developer then wants something to happen: 1. The user interacts with the application. 2. The object on which the user interacts dispatches an event (for example, when a button has been clicked). 3. Another object is listening for that event and reacts when the event occurs. 4. Code inside the listening object is executed
Analyzing a Simple Example Let’s examine a concrete example: A user clicks a button and text appears in a label. The following code makes this happen.
A button appears with the label “Click Me”. When the user clicks the button, the click event is dispatched. In this case the ActionScript code myL.text=’Button Clicked’ is executed. The text property of the label is assigned the Button Clicked string value.
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NoTe: There are nested quotes in this example . The double quotes surround the code for the click event, and the nested single quotes delineate the string . This can become difficult to read
and, as you will see in the next section, is not the ideal way to write code .
Until now, when you have assigned values to an attribute in MXML, you have supplied one of two types: scalar values or bindings. Scalar values are simple data types like String, Number, or Boolean values. You have used these when setting x and y values, widths, and label text values. You have also used bindings for properties. This was done whenever you used braces ({}) in a value. Remember from the last lesson that the braces let you enter ActionScript for the property value. When supplying a value to an MXML attribute that represents an event (that is, the click event in the previous code), the Flex compiler is smart enough to implicitly understand that the string inside the quotes is ActionScript. That’s why you can enter ActionScript directly for the click event, click=”myL.text=’Button Clicked’”, without using the braces you used in the previous lesson. Just as code hinting assisted you when you were entering property names, so code hinting will assist with event names. In the following figure, you see the clear and click events displayed with the lightning bolt icon in front of them, which designates events.
Handling the Event with an ActionScript Function The code in the last example successfully sets the text of the Label object when the Button is clicked. However, a problem with this approach soon develops when you want to execute more than one line of ActionScript when the event occurs. To do this, you could place many separate chunks of code inside the quotes for the click event, each separated by a semicolon. Although this works, it is messy and far from a best practice. Also, you may want the same lines of code to be executed when several different events occur. In the approach shown
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earlier, you would have to copy and paste the same code into several places. That can become a big nightmare if you ever need to edit that code, as you now need to find and edit each copy. A better approach is to handle the event in an ActionScript function. The function will be built in an block that simply tells the Flex compiler that the code in the Script block is ActionScript. So instead of placing the actual ActionScript to be executed as a value for the click event, you will call a function instead. Following is a refactoring of the code examined earlier, using the best practice of placing the code to be executed in a function.
NoTe: The block inside the Script block marks the section as character data . This tells the compiler that the data in the block is character data, not well-formed XML, and that it should not show XML errors for this block . Now when the Button is clicked, the function clickHandler() is called, and the string is written to the label. In this case, because no quotes were nested, you can use double quotes around the string in the function. The function has a return type of void. This means that the function will not return a value. It is a best practice to always specify the return type of functions you build, even if you use void to indicate that no data will be returned. The compiler will give you a warning if you do not specify a return type on a function. It is best to heed those warnings, as specifying types enables the compiler to ensure that you don’t make simple typos, like assigning a variable that is supposed to contain a Button to something that is supposed to contain a Number.
Passing Data When Calling the Event Handler Function You may wish to pass data when calling the function. This works in ActionScript just as you’d expect. You place the data to pass inside the parentheses following the function name, and
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then modify the event handler to accept the parameter. Just as you will always specify a return type on your function, so will you need to specify the type for any parameter that the function will accept. In the following code example, the string to be displayed in the label is passed to the clickHandler() when the button is clicked.
In this case, when the Button is clicked, the string Value Passed is sent to the event handler function. The function accepts the parameter in the toDisplay variable, which has a data type of String. The value stored in the toDisplay variable is then displayed in the label’s text property.
Using Data from the Event Object .
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In practice, subclasses of Event are used much more often than the Event class. Imagine a situation where you drag an item from one place on the screen to another. Knowing that an item was dragged, and where it was dragged to, are both important, but you would likely want some additional information as well: what item was being dragged, for example, and what the x and y positions of the mouse were when the item was dropped. To provide this more specific information, Event subclasses and additional properties are added, meaning you will often interact with event types such as DragEvents or ResultEvents. The following figure from the documentation shows how many other event classes are based on, or subclassed from, the generic Event object.
Examine the following code that sends an event object, in this case a MouseEvent object, to the event handler.
In the code, an event is passed to the event handler, and the word click will be displayed in the Console view when the application is debugged. You are now going to refactor the FlexGrocer application to use a function for the View Cart buttons. 1 Open the FlexGrocer.mxml file that you created in the previous lesson. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson05/start folder.
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Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Directly below the closing tag, insert a new tag pair. When you do so, Flash Builder will automatically insert a CDATA (character data) block for you. Your code should look like the following:
MXML files are XML files. Some of the characters you are about to use when writing ActionScript code are not allowed inside XML directly. The CDATA block instructs XML parsers to treat the data inside it differently, allowing these characters. Throughout this book you will be asked to add functions and variables inside the Script tag. You should always add these inside the CDATA block. 3 Inside the Script block (remember that also means inside the CDATA block), add a new private function named handleViewCartClick(). This function will accept a single argument named event of type MouseEvent and return nothing (have a return type of void). private function handleViewCartClick( event:MouseEvent ):void { }
As you can see, the first word in this function declaration is private. This is the function’s scope. Here you are indicating that a function is accessible only from inside this class. You are specifying that a single argument will be passed to this function and that argument will be of type MouseEvent. NoTe: You will deal more with the scope of functions later in this book . However, there isn’t enough room in this book to cover both object-oriented programming and Flex, so if you are uncomfortable with this or any other object-oriented concepts, please review any number of excellent books on that topic, or read the extensive entries on Wikipedia .
4 Inside the handleViewCartClick() function, add the ActionScript code to change the currentState property to cartView. private function handleViewCartClick( event:MouseEvent ):void { this.currentState=”cartView”; }
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5 Find the Button with the id btnCartView inside the controlBarContent. Currently that Button sets the currentState directly. Instead, change this tag so it now calls the handleViewCartClick() function and passes the event object.
6 Find the Button inside the cartGroup with the label View Cart that currently sets the currentState to cartView directly. Change this tag so it now calls the handleViewCartClick() function and passes the event object.
7 Save the file and click Run. As with all refactoring, the application should behave the same as it did previously with both View Cart buttons taking the application to the cartView state.
Inspecting the Event Object In this section, you will use the debugger to examine MouseEvent properties. Learning to use the event object and its properties is one key to writing reusable code in Flex. 1 Add a breakpoint on the closing parenthesis of the handleViewCartClick() function by double-clicking in the marker bar just to the left of the code and line numbers. A small blue dot will appear in the marker bar indicating where the program execution will halt. You will be able to examine values at this point.
The debugger is immensely helpful in understanding what is happening in Flex. Use it often to get a sense of what is going on “under the hood” of the application. Tip: Event handlers should be named consistently . For instance, in this lesson you’ve seen a click event on two View Cart buttons handled by an event handler named handleViewCartClick() . There is no “right” way to name event handlers, but you may wish to pick a naming convention and stick with it . The most important point is make them as descriptive as possible .
2 In the Flash Builder interface, click the down arrow next to the Debug button and choose FlexGrocer.
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Tip: If you have multiple application files, such as FlexGrocer and Checkout, you can choose the specific one you wish to run by clicking the down arrow next to the Run or Debug buttons instead of the button itself .
Debug
3 In the browser, click either of the buttons labeled View Cart. In Flash Builder, you will be prompted to use the Debug perspective. You should select it.
Clicking Yes will switch your Flash Builder view to the Debug perspective, which is optimized with the views needed to debug an application. 4 Double-click the tab of the Variables view. This will cause the Variables view to maximize and take up the full screen. The Variables view can provide an immense amount of information. A full-screen view will make navigating that information easier. You will see two variables displayed, this and event, as shown here:
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Right now, the this variable represents the entire application. When you learn about creating your own components in future lessons, you will learn that this always represents a context that can change. If you click the arrow in front of the variable, you will see many properties and associated values. The event variable represents the event variable local to the function where the breakpoint was placed. The letter L in a circle in front of event indicates it is a local variable. 5 Click the outline of the arrow to the left of the event variable and then the arrow to the left of the [inherited] set of properties. Locate the target property. Notice that you clicked a button to get here and that the target of this event is the Button component that broadcasts the event. Also notice the type property has the value click. From the earlier discussion, neither of those property values should be a surprise. In this case, the values listed in the [inherited] section are those available to every event because they come from the superclass. The properties listed outside that section are the specific properties available on the MouseEvent. Note that some of these properties, such as altKey or localX, wouldn’t make sense to have on every event in the system, but they are welcome properties on a MouseEvent. 6 Click the arrow to the left of the target, then click the arrow to the left of the [inherited] set of properties. Locate the id property.
This property will depend on which View Cart button you clicked. If you clicked the button in the control bar, then the property’s value is btnCartView, which is what you assigned in the code. If you chose the View Cart button in the shopping cart area, then
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you will see an assigned id such as _FlexGrocer_Button5. If you wish, repeat the steps to view the id values for the other Button. All UI objects in Flex have an id. If you don’t assign one, then Flex will. 7 Double-click the Variables tab again to restore it. Click the red box on either the Debug or Console view to terminate the debugging session. Don’t forget to terminate debugging sessions. It is possible to have one debugging session running alongside another in certain browsers. You might want to do this in special cases, but not normally. 8 Return to the Development perspective by clicking the chevron (>>) in the top right of your screen and then choosing Flash. Tip: If you place the cursor to the left of the Open Perspective icon, the sideways arrow will appear . You can drag to the left to increase the space allotted for perspectives . You will be able to see both the Debug and Flash perspectives and will be able to click their tabs to switch between them . 9 Remove the breakpoint on the closing parenthesis of the handleViewCartClick() function by double-clicking the blue dot in the marker bar just to the left of the code and line numbers.
Handling System Events As mentioned earlier, there are two types of events you handle in Flex. First, there are user events, like the MouseEvent that you handled in the previous section. Second, there are system events that are dispatched by the Flex framework in response to a change in an internal condition. In this section you will see one of the most common system events related to startup and understand a bit about its use.
Understanding the creationComplete Event The creationComplete event is one of many useful events dispatched by Flex components. This event is dispatched when a component has been instantiated and knows both its size and position in the application. The creationComplete event of a parent component is always dispatched after all its children have dispatched their creationComplete events. In other words, if you have a Group with several Buttons inside it, each of the Buttons will dispatch its creationComplete event before the Group dispatches its own creationComplete event.
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Let’s examine the following code snippet: creationComplete="addToTextArea('Label creationComplete')"/>
First, look at the event handler named addToTextArea. This event handler simply accepts a parameter named eventText and places it in a TextArea, followed by a return so the text doesn’t all run together. In each component, which includes Application, HGroup, Label, and Button, a creationComplete event is handled. When each component finishes its completion process, the event is dispatched and the corresponding string is sent to the event handler for display in the TextArea. Flex begins creating components from the outside, working its way in. However, means a component has been instantiated and knows both its size and position. In Flex, the size of a component (such as the HGroup) is often dictated by the collective
creationComplete
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size of its children. Therefore, the Label and Button must be finished (and must have dispatched their creationComplete events) before the HGroup can be considered complete and can dispatch its own event. When all the Application’s children are created, the Application dispatches its creationComplete event. The results displayed in the TextArea appear as shown here:
Armed with this knowledge, you can probably understand why the creationComplete event of the Application object is often used for doing work such as modifying or retrieving data.
Modifying Data on Creation Complete Currently your FlexGrocer project uses data binding to populate a RichText control with data from an XML model. Your code looks like this: … Dairy Milk assets/dairy_milk.jpg 1.20 1.99 true true Direct from California where cows are happiest! … …
Flex knows to automatically populate the RichText control with the data retrieved from the description property of the groceryInventory object. In this section, we will use the creationComplete event to make a small modification to the description in ActionScript and see that the RichText control displays the modified data.
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1 Open the FlexGrocer.mxml file that you modified in the previous section. If you didn’t complete the previous section, you can import FlexGrocer-PreCreationComplete.fxp project from the Lesson05/intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Inside the Script block, just below the existing handleViewCartClick() function, add a new private function named handleCreationComplete(). The function will accept a single argument, named event of type FlexEvent, and return void. private function handleCreationComplete(event:FlexEvent):void { }
While you are typing FlexEvent, Flash Builder will try to provide possible choices as you type. If you choose one of the items on the pop-up menu (or use the arrow keys and press Enter on the correct option), Flash Builder will complete the name for you and perform one other very important step, importing the class.
3 If you choose one of the options on the pop-up menu, Flash Builder adds an import line to the top of your Script block. This line is an import statement that lets Flash Builder know where the class you are referencing resides. You can think of import statements as more or less the ActionScript equivalent of the namespaces you used in MXML: import mx.events.FlexEvent;
If you do not have this line in your file, you have two options: You can place your cursor right after the closing t in FlexEvent and press Ctrl+Spacebar. This will cause Flash Builder to open the code-completion pop-up again. If there is only one matching option, Flash Builder automatically selects it and adds the import for you. Alternatively, you can just type the import statement just inside the Script tag (remember that also means inside the CDATA block).
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4 Inside this function, you will assign the string “Cheese from America’s Dairyland” to the description property of the groceryInventory object. private function handleCreationComplete(event:FlexEvent):void { groceryInventory.description = “Cheese from America’s Dairyland”; }
This statement will replace the original text of the description property with your new text as soon as the Application dispatches its creationComplete event. 5 Inside the Application tag, you will instruct Flex to call the handleCreationComplete() function when the creationComplete event occurs, passing it the event object. Your code should read like this:
6 Save and run the application. When you move your mouse over the bottle of milk, you should see the new description appear.
In this simple example, you have handled the creationComplete event to modify data and allowed data binding to provide the changed data to the RichText control for display.
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What You Have Learned In this lesson, you have: • Gained an understanding of event handling in Flex (pages 94–97) • Learned to pass arguments to an event handler (pages 97–98) • Refactored the application to use an ActionScript event handler (pages 98–104) • Handled a creationComplete event (pages 104–108)
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LESSON 6
What You Will Learn In this lesson, you will: • Externalize your data • Distinguish embedded and loaded data • Create an HTTPService object that returns data as Objects • Understand security issues involved with retrieving data into Flash Player • Search XML with E4X expressions • Create an HTTPService object that returns data as XML • Build an XMLListCollection from your dynamic XML • Display your data in a List
Approximate Time This lesson takes approximately 1 hour and 30 minutes to complete.
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Lesson 6
Using Remote XML Data In this lesson, you will begin to connect the FlexGrocer application to XML data. First, you will use a local XML file that you will embed in your application. This will demonstrate one technique used to separate your XML data into a separate file. Then you will use the HTTPService class to load remote XML data into the Application. In this context, the word remote means that the data is remote to the application: in other words, not embedded. The data can exist on a remote server or in external files on the same server, but in either case the data is transmitted through HTTP. You will work with this XML data in several formats, including functionality from the ECMAScript for XML (E4X) implementation that allows you to use XML as a native data type in ActionScript (it’s built into Flash Player just like Number, Date, or String). Controls can be populated with this data to easily display complex datasets and enable the user to navigate the data quickly. Examples of these controls include List, ComboBox, and Tree. You will be using the List control in this lesson.
You will use XML Data to populate the application with live data.
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Using Embedded XML In this task, you will make two major changes: externalizing your data (defining it somewhere external to your Application class), and treating it as XML at runtime. Currently the XML for your Milk product is hard-coded directly into the Application class in an tag. Hard-coding XML is extremely convenient when you’re prototyping new code, but it clutters up your application and can make it difficult to see the important code as new and different types of data are needed.
Externalizing the Model Your first task is to externalize the data in your application and reference it by filename. 1 Open the FlexGrocer.mxml file. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson06/start folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Make sure Flash Builder is in Source view. To switch between Design view and Source view in Flash Builder, click the buttons in the menu bar near the top of the window. 3 Expand the FlexGrocer project and the src folder inside the Package Explorer. Right-click the assets folder and choose New > File.
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NoTe: If you are using a Mac, please excuse our PC-centric use of the term right-click and use Control-click instead .
4 Enter inventory.xml in the File name field. Click Finish. You will move the XML specified in your Application file to the external file you just created (inventory.xml) and declutters your Application code.
5 On the first line of the new file, you need to add an XML document type declaration that specifies that this file is a version 1.0 XML document encoded with utf-8.
This is the same declaration used at the top of each of your MXML files. Should you not feel like typing it, you can copy it from the first line of the FlexGrocer.mxml file. 6 Copy the groceries XML node from within the Model tag of your application. This is the XML that starts with and ends with , including both of those tags. 7 Paste that XML node into the inventory.xml file directly below the document type declaration. Your inventory.xml file should look like this: Dairy Milk assets/dairy_milk.jpg 1.20
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1.99 true true Direct from California where cows are happiest!
8 Save the inventory.xml file. You will now use this external file in place of the hard-coded XML in the application. 9 Switch back to your FlexGrocer.mxml file and delete the groceries XML from inside the Model. The tag should now be empty.
10 Inside the Model tag, add a source attribute and set it to assets/inventory.xml.
Specifying the source here tells the Model tag to use your external file as its model. 11 Finally, change the Model tag to be self-closing. You now want the content in the source file, not content inside the Model tag, to be used for the inventory data.
12 Save and run your application. The product description and information should work just as they did before; however, the data is now coming from the inventory.xml file.
Choosing Between Objects and XML When working in Flex with XML data, you have two choices: Work directly with the XML or convert the XML to an object, then use that object instead of the XML. The Model tag you have been working with so far does the latter. It is using the XML you typed into the inventory.xml file as a template, from which it creates a series of generic objects. In this exercise you will see this structure in the debugger and then change the code to use XML directly. 1 Open the FlexGrocer.mxml file. Alternatively, if you didn’t complete the previous exercise or your code is not functioning properly, you can import the FlexGrocer-PreXMLTag.fxp project from the Lesson06/ intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve.
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2 Add a breakpoint inside the handleCreationComplete() method by double-clicking in the marker bar just to the left of the code and line numbers. A small blue dot will appear in the marker bar indicating where the program execution will halt.
3 Debug the FlexGrocer application.
In Flash Builder, you will be prompted to use the Debug perspective. Click Yes. Flash Builder will stop on the line where you set a breakpoint. 4 Select groceryInventory and then right-click it. Choose Create Watch Expression.
Flash Builder will add groceryInventory to the Expressions view. If you cannot find the Expressions view, go to Window > Expressions.
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5 Expand the groceryInventory object by clicking the triangle to the left of it in the Expressions view. Note that the item type is mx.utils.ObjectProxy.
ObjectProxy instances are a special type of wrapper for Objects. They effectively enable Objects to dispatch events, which is important for data binding, as you will learn in future lessons. The Model tag converted your XML to Objects. This is actually its intended purpose, to provide the developer with a quick way to define potentially hierarchical objects using XML. As mentioned previously, this is one of two ways that you can choose to deal with XML in Flex. The other is to leave it as XML and manipulate it directly. You will do that next. 6 Click the red square to terminate this debugging session. 7 Change the tag to an tag:
8 Debug the FlexGrocer application again. Flash Builder will stop on the line where you set a breakpoint. 9 The watch expression you set for groceryInventory is still active, so if you expand the view of this object in the Expressions view, you will see that it is no longer an ObjectProxy, but now XML.
10 Click the Resume button (the green Play icon to the left of the red Terminate button), and your application will continue to run in the web browser.
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11 In your web browser, move the mouse over the milk image, and you will see that the description appears as it did before. The description still displays thanks to a feature of Flash Player called ECMAScript for XML (E4X). It allows you to access data inside XML in much the same way you access other objects, greatly simplifying the task of using XML and making your existing code able to use either objects or XML. 12 Terminate your debugging session by clicking the red Terminate button. Remove your breakpoint before continuing. Your application is now using XML directly instead of Objects for this data. The decision to use one or the other inside Flex is a recurring theme, as you will see in the following exercises.
Using XML Loaded at Runtime In this previous exercise you used an external XML file both as an Object and as XML to provide data for your application. However, in both cases, the data was embedded in the application. In other words, the data in your external inventory.xml file became part of the final SWF file produced by Flash Builder. If the data in your inventory.xml file changes, you will need Flash Builder to recompile your FlexGrocer application before the changes become available to someone running the application. This is fine for demo software and potentially even certain pieces of data that do not change very often (for example, the states in the United States). However, it is not practical for most cases. In this section you will learn to load data from an external source using the HTTPService so that your data can change independent of your application. Simply stated, the HTTPService component allows your application to use data it retrieves at a given URL. By default, the data will be returned in an Object format (as it was when you embedded your file with the Model tag). You can choose the format in which you wish to use the data (for example, Object, XML, or text). In this section you will use the returned data in both the Object and XML formats. The general steps for using HTTPService follow: 1. Create an HTTPService object. 2. Invoke the send() method of the object. 3. Use the returned data.
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Creating an HTTPService Object .
Invoking the send() Method When you wish to retrieve data from a given URL, you must send a request for that data. The HTTPService object contains a method to send this request, named send(). When you create the HTTPService and specify the URL, the HTTPService is ready to retrieve your data; however, it will not begin this process until you invoke the send() method. In many cases, you will want to retrieve data at application startup. In this lesson, we will use the creationComplete event of the Application tag to retrieve remote data.
Accessing the Returned Data Data retrieved from the HTTPService can be accessed in two ways.
lastResult The first is to access the data directly via the lastResult property of the named HTTPService object. To get to the data, build an expression with the following elements: • The instance name of the HTTPService • The lastResult property • The dot path into the data you are trying to access For example, in the next exercise you have an HTTPService defined as
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and it will retrieve the following XML: Bunch 4 Dozen 2 Each 1 Pound 3
By default, it will turn that XML into a series of objects. So, to access the unit node data via the lastResult property, you would use the following code: unitRPC.lastResult.allUnits.unit
This is the instance name of the HTTPService (unitRPC), followed by the lastResult property, followed by the path to the piece of data you care about (unit). This method of accessing the returned data is a useful learning tool, so you will see another example later in this lesson, but in real applications you will rarely use this method because it can be clumsy and confusing. In practice, you will access the data via an event object inside an event handler.
result If you are using an HTTPService defined as follows,
then the unitRPCResult() handler will be called when the XML is successfully retrieved. The proper method signature for this result handler is private function unitRPCResult(event:ResultEvent):void{ }
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You can access the unit data in the body of the function by specifying event.result.allUnits.unit
The returned data is available in the result property of the event. To reiterate, the two ways of accessing data returned from this HTTPService are which can be used anywhere in the application, or event.result, which can be used only inside the event handler.
unitRPC.lastResult,
Being Aware of Security Issues The Internet is not a secure place. It is full of people “borrowing” information and attempting to access content they have no right to view. As Flash Player and your application must live in this world, many security restrictions are placed on what Flash Player is allowed to do on your system and how it is allowed to access data. The restriction that we are most concerned with in this section pertains to loading data from a remote server. Flash Player uses a concept called sandboxes at the core of its security model. You can visualize this as a literal sandbox. All the children sitting in a single sandbox are allowed to interact and play with each other’s toys. However, when a child from another sandbox wanders over to play or retrieve a toy, he is met with scrutiny and distrust. Internally, Flash Player keeps all the content from different domains (mysite.com versus yoursite.com) in different sandboxes. As part of the security measures imposed on these sandboxes, content loaded from one domain is not allowed to interact with content loaded from another domain. Following this logic, if your application is at http://www.yoursite.com/yourApp.swf and it attempts to load an XML file at http://www.flexgrocer.com/units.xml, it will be denied and a security error will occur as these two items exist in different security sandboxes. The Flash Player security model requires the owner of flexgrocer.com to allow you access to that data, or you will be denied by default. The owner of that domain can allow such access by creating a cross-domain policy file. This file, named crossdomain.xml, specifies which domains have access to resources from Flash Player. The file is placed on the root of the web server that contains the data to be accessed. Here is an example of a cross-domain policy file that would enable your application on www.yoursite.com to access the units.xml file on flexgrocer.com. The file would reside in the web server root of flexgrocer.com:
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You can also use wildcards in a cross-domain policy file. This example allows anyone to access data:
Tip: Browse the URL www .flexgrocer .com/crossdomain .xml to see the cross-domain file that allows you to retrieve data for this book . Also check www .cnn .com/crossdomain .xml to see who CNN allows to syndicate their content using Flash Player . More information about the sandbox restrictions of Flash Player is available in the tech note on the Adobe site at www.adobe.com/cfusion/knowledgebase/index.cfm?id=tn_14213, along with a complete description of the cross-domain policy file, which can be found at www.adobe.com/devnet/articles/crossdomain_policy_file_spec.html. Before deploying a cross-domain security file like this on a server, make sure you understand all the ramifications.
Retrieving XML Data via HTTPService In this exercise, you will use an HTTPService object to retrieve data that contains the categories for grocery items—such as Dairy or Meat. You will use the debugger to make sure that the data is returned correctly and will verify that you can see the data in the event object. 1 Open a web browser and go to the following URL: http://www.flexgrocer.com/category.xml Notice the structure of the XML. This is the data you will retrieve using the HTTPService. Dairy 4 Deli 5 Fruit 3
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Meat 1 Seafood 6 Vegetables 2
2 Open the FlexGrocer.mxml file. Alternatively, if you didn’t complete the previous exercise or your code is not functioning properly, you can import the FlexGrocer-PreHTTPService.fxp project from the Lesson06/intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 3 Inside the block, directly below the tag, add an tag. Give it an id of categoryService, and specify http://www.flexgrocer.com/category.xml as the url property. Specify a result handler named handleCategoryResult and be sure to pass the event object, as follows:
You are specifying the URL of the HTTPService to point to the XML you examined in step 1. In the next step, you will write an event handler with the name handleCategoryResult(), which will be called when the data has been retrieved. 4 In the Script block below the existing functions, add a new private function with the name handleCategoryResult() that returns void. The method will accept a parameter named event, typed as a ResultEvent. At this point the function is empty. private function handleCategoryResult(event:ResultEvent):void{ }
If you chose ResultEvent in the pop-up list when you were typing, or if you pressed Enter when the ResultEvent was selected, then Flash Builder automatically added an import statement for you near the beginning of the Script block. If you do not see
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import mx.rpc.events.ResultEvent; near the beginning of your Script block, then you must add it manually now. Learning to use the code completion features of Flash Builder as soon as possible will save you hours in the course of this book and thousands of hours in your lifetime as a Flex developer.
5 Find the handleCreationComplete() method you wrote in Lesson 5, “Handling Events.” This method is called when the creationComplete event of the application is dispatched. Presently this method changes some data in the groceryInventory object. Delete the line that reads groceryInventory.description = “Cheese from America’s Dairyland”;
from inside your method. You will now replace this with code to request data. 6 Call the send() method of the categoryService object inside the handleCreationComplete() method. Your method should read as follows: private function handleCreationComplete(event:FlexEvent):void { categoryService.send(); }
The categoryService object is the HTTPService that you defined in your declarations. Invoking its send() method asks Flash Player to go out on the network and request the data found at http://www.flexgrocer.com/category.xml. NoTe: This is one of the most misunderstood aspects of programming in Flex . Data retrieval is asynchronous . Just because you called the send() method does not mean that your XML data is ready to use . The act of calling send() starts the process of getting that data . However, just as your web browser takes a moment to load a page, so this data will not be available and ready to use until the result event occurs .
7 Add a breakpoint on the closing bracket of the handleCategoryResult() method by doubleclicking in the marker bar just to the left of the code and line numbers. A small blue dot will appear in the marker bar indicating the spot where program execution will halt. Placing a breakpoint here gives you the chance to examine the data returned by the HTTPService. 8 Debug the application. Return to Flash Builder and make sure you are in the Debugging perspective. Double-click the Variables view tab. Drill down to the returned data by clicking the plus sign in front of event > result > catalog > category. Here you see the six category values in brackets [0], [1], [2], [3], [4], and [5] when you expand them. If you dig far enough into the structure, you will eventually see categories such as Fruit, Meat, and Dairy.
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9 Double-click the Variables view tab to return it to its normal size. Terminate the debugging session by clicking the red Terminate button in the Debug or Console view. Finally, return to the Development perspective. You have now used an HTTPService object to retrieve data, and you used debugging techniques to confirm that it has been returned to the application. Soon, you will put the data to use.
Searching XML with E4X .
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Working with E4X Operators In this task and through the rest of this lesson, you will use E4X functionality. The new E4X specification defines a new set of classes and functionality for XML data. These classes and functionality are known collectively as E4X. First, for a very basic, very quick review of XML terminology, examine the XML object as it would be defined in ActionScript: private var groceryXML:XML = new XML(); groceryXML= bag Cleaned and bagged pound Baby carrots, cleaned and peeled each Sweet Fuji pint Firm and fresh pint Deep red and juicy ;
The following statements describe the XML object, with the XML terminology italicized: • The root node is catalog. • There are two category nodes, or elements; for our purposes these will be synonyms. • The product node has two child nodes (children), called unit and desc. • The product node has two attributes, name and cost.
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NoTe: If you scrutinize the XML in more detail, you will also see a node with both berry-related products nested inside . This is done intentionally to show the power of the E4X operators in the examples to follow .
One last concept that you must understand before continuing is the difference between XML and an XMLList. Put simply, valid XML always has a single root node. An XMLList is a list of valid XML nodes without its own root node. For example:
represents valid XML. Further, each of the nodes is a valid piece of XML in and of itself. Conversely, the following structure:
does not have a single root node and is not valid XML. It is however, a list of valid XML nodes, and we refer to this construct as an XMLList. This XMLList has a length of 3 as there are three nodes immediately inside it. Finally, if we were to examine the following XML:
We could say this is valid XML, as it has a single root node, and it is a valid XMLList of length 1. All the E4X operators you are about to learn return XMLLists as their output type. Now that you understand the basic XML terminology, you can start using some of the powerful E4X operators. A small application has been written for you to test some of these operators. 1 Import the E4XDemo.fxp project from Lesson06/independent folder into Flash Builder. Please refer to Appendix A for complete instructions on importing a project. A new project will appear in Flash Builder. 2 Inside the new project, open the E4XDemo.mxml file. 3 Run the E4XDemo application.
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On the top left you see the XML shown earlier in this lesson. This application allows you to search that XML by entering an E4X expression into the text input on the bottom left and clicking the Apply e4x Expression button. The right side will display the resulting XMLList from that operation in two forms, as a tree of data on the top and as a formatted string on the bottom. 4 Click the Apply e4x Expression button to apply the default expression category.product. NoTe: In E4X expressions, the root node (in this case catalog) is part of the document and not used in statements .
This expression uses the dot (.) operator. This is one way to access data in the XML document. The dot operator behaves much like the dot in object.property notation, which you are familiar with. You use the dot operator to navigate to child nodes. The expression yields the following results: bag Cleaned and bagged pound Baby carrots, cleaned and peeled
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each Sweet Fuji
In this case, the expression category.product indicates that you want all product nodes that are directly under category nodes. What was returned is an XMLList. Notice that the products that are children of the berries node did not appear.
5 Now enter the expression category.product.unit and click the button to apply it. Here the dot operator again navigates the XML and returns the unit node for the three products retrieved in step 4. bag pound each
6 Enter category.product[1] and apply the expression. This demonstrates that you can apply array notation in E4X. Here you get the second product because the list is zero indexed. pound Baby carrots, cleaned and peeled
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This again shows that E4X lets you use familiar notation to work with XML. In previous versions of ActionScript, you had to use specific methods to access data in XML. 7 Enter category.product.(unit==”bag”) and apply the expression. This limits the returned products to those whose unit node is bag. You’ve limited the data returned by putting a filter in the expression. bag Cleaned and bagged
The parentheses implement what is referred to as predicate filtering. 8 Enter category.product.(@cost==”1.95”) and apply the expression. Two product nodes are returned. bag Cleaned and bagged each Sweet Fuji
You have now performed predicate filtering on an attribute—hence the use of the attribute operator (@) in the parentheses (@cost==”1.95”). Also notice that if multiple nodes match the filter, you simply get multiple nodes returned—in this case both the lettuce and apples products. 9 Enter category.product.(@cost==”1.95”).(unit==”each”) and apply the expression. This expression demonstrates that you can apply predicate filtering multiple times. This results in only one product being returned. each Sweet Fuji
10 Finally, to see the berry products get involved, enter category..product as the expression. You see that all products are returned, regardless of where they are in the XML. bag Cleaned and bagged
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pound Baby carrots, cleaned and peeled each Sweet Fuji pint Firm and fresh pint Deep red and juicy
This is an example of the very powerful descendant operator, represented by two dots (..). This operator navigates to the descendant nodes of an XML object, no matter how complex the XML’s structure, and retrieves the matching nodes. In this case the descendant operator searched through the entire XML object and returned all the product nodes. 11 Enter category..product.(@cost>2) and apply the expression. This combines two operators and returns three products. pound Baby carrots, cleaned and peeled pint Firm and fresh pint Deep red and juicy
Here both predicate filtering and the descendant accessor are in use. E4X searched all the XML, regardless of position, and found three matches. 12 Close the E4XDemo project by right-clicking the project name and choosing Close Project. You have now seen a slice of the very powerful E4X implementation in ActionScript 3.0. For more information, see “Working with XML” in the Programming ActionScript 3.0 documentation that comes with Flex. You can now return to the FlexGrocer project and begin working with dynamic XML.
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Using Dynamic XML Data
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Using Dynamic XML Data As a Flex developer, you will have the opportunity to work with both XML and Objects. Over time you will decide which works better for your project in a specific situation. Starting with the next lesson and through the remainder of the book you will convert your XML to strongly typed objects. For the remainder of this lesson, however, you are going to work strictly with XML to display a list of categories. While the techniques in this book are often presented in an improving fashion, meaning that those later in the book are often more-functional refactorings of earlier work, that is not the case with XML and Objects. These are simply two different techniques, each with advantages and disadvantages, that can be used to solve a problem. As you saw in the previous exercise, XML is a quick and flexible way to present data. E4X expressions allow it to be searched and manipulated extremely quickly, and this makes it very powerful. However, as you will no doubt discover, an application can fail due to a simple typographical error, something that typed objects can resolve at the expense of flexibility. Currently the HTTPService tag in your FlexGrocer application is defaulting to returning dynamic Objects instead of XML when retrieving data. You are going to modify this property as well as store the returned data in an XMLListCollection for future use. You will be working with the data retrieved from http://www.flexgrocer.com/category.xml. The structure of that XML file is listed below for your reference in this exercise. Dairy 4 Deli 5 Fruit 3 Meat 1
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Seafood 6 Vegetables 2
1 Open the FlexGrocer.mxml file. Alternatively, if you didn’t complete the previous exercise or your code is not functioning properly, you can import the FlexGrocer-PreXMLCollection.fxp project from the Lesson06/intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Inside the block, find the tag. Add a new property to this tag named resultFormat and specify the value as e4x:
The resultFormat property tells the HTTPService how it should provide any data retrieved from this request. By default it returns data as dynamic Objects wrapped in ObjectProxy instances. Changing this format to e4x instead provides you with XML that you can manipulate using E4X operators. 3 Make sure that you have a breakpoint set on the closing bracket of the handleCategoryResult() method. 4 Debug the application. Return to Flash Builder and make sure you are in the Debugging perspective. Double-click the Variables view tab. Drill down to the returned data by clicking the plus sign in front of event > result > catalog. Here you see the six category values all represented as XML. Expanding any of these nodes will provide you with more detail.
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5 Double-click the Variables view tab to return it to its normal size. Terminate the debugging session by clicking the red Terminate button in the Debug or Console view. Finally, return to the Development perspective. 6 Near the top of your Script block, just under the import statements, add a new private variable named categories of type XMLListCollection. If you used code completion, Flash Builder has already imported the XMLListCollection for you. If you did not, then add an import for mx.collections.XMLListCollection before continuing. XMLListCollection is a special class that holds and organizes XMLLists. It allows for those XMLLists to be sorted and filtered. You will learn about collections in the next lesson. 7 Directly above the variable you just created, you are going to add a metadata tag to indicate that the variable is bindable. Type [Bindable] directly above the variable definition. [Bindable] private var categories:XMLListCollection;
The Bindable metadata tag tells Flex to watch this particular collection for changes. In the event of a change, the Flex framework should notify everyone using this data so they can update and refresh their display. You will continue to learn about this powerful feature as you progress through the book. 8 Inside the handleCategoryResult() method, you need to instantiate a new XMLListCollection and assign it to the categories variable you just created. private function handleCategoryResult( event:ResultEvent ):void { categories = new XMLListCollection(); }
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After this line of code executes, the categories variable will contain a new XMLListCollection. However, that collection does not yet contain your data. 9 Pass the E4X expression event.result.category into the constructor of the XMLListCollection. private function handleCategoryResult( event:ResultEvent ):void { categories = new XMLListCollection( event.result.category ); }
This expression will return all the categories immediately inside the XML returned from the HTTPService call. By passing this to an XMLListCollection constructor, you are providing a way to further manage this data at runtime. 10 Make sure you have a breakpoint set on the closing bracket of the handleCategoryResult() method. 11 Debug the application. Return to Flash Builder and make sure you are in the Debugging perspective. 12 Select the word categories and then right-click it. Choose Create Watch Expression. Flash Builder will add categories to the Expressions view. If you cannot find the Expressions view, go to Window > Expressions. 13 Expand the categories object by clicking the triangle to the left of it in the Expressions view.
The Expressions view says that the type of item is an mx.collections.XMLListCollection. Inside the XMLListCollection, you will find items denoted by array syntax. Expanding these items will reveal each of your categories. 14 Remove all the items from the Expressions view by clicking the Remove All Expressions button (the double X) to the right of the word Expressions. Tip: At any time you may remove all the items from the Expressions view by clicking the double X or just a single item by highlighting it and clicking the X . 15 Terminate your debugging session by clicking the red Terminate button and remove your breakpoint before continuing.
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Using the XMLListCollection in a Flex Control
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Using the XMLListCollection in a Flex Control Your application now retrieves data from an HTTPService and stores it as an XMLListCollection. However, presently the only way to ensure that the application is working is to use the debugger. In this exercise you will display the category data in a horizontal list across the top of the application. 1 Open the FlexGrocer.mxml file. Alternatively, if you didn’t complete the previous exercise or your code is not functioning properly, you can import the FlexGrocer-PreList.fxp project from the Lesson06/intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Add an control inside the controlBarContent section of your Application. You can add this immediately after the existing Buttons.
3 Specify that the List will remain 200 pixels from the left side of the controlBar and will have a height of 40 pixels.
4 Specify that the List will use a HorizontalLayout.
Previously you used horizontal and vertical layouts for groups, but List classes can also use these same layout objects to determine how their children should be arranged. 5 Now indicate that the dataProvider property of the List instance should be bound to the categories variable you defined and populated earlier.
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This syntax tells the Flex framework that, in the event the categories property changes, the list will need to be provided with the new value so that it can react. You will work extensively with List and dataProvider in future lessons. 6 Save and run the application.
Your new list runs across the top of the page, with the elements arranged horizontally. Unfortunately, instead of displaying category names, you are now displaying the XML associated with the category. Notice that the data you really want displayed is in the node of the category XML. 7 Return to the FlexGrocer application and add a new property to your List called labelField. Set this property equal to name.
The labelField property tells the list which field (property) inside your data to use as the label for the list item. 8 Save and run the application.
You now have a much more reasonable-looking list of category names that you will continue to use in the next lessons.
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What You Have Learned
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What You Have Learned In this lesson, you have: • Externalized your data as an XML file (pages 112–114) • Used the external data first as an object and then as XML (pages 114–117) • Loaded remote XML data (pages 117–124) • Learned about security sandboxes with remote data (pages 120–121) • Explored E4X operators (pages 124–131) • Used an XMLListCollection with your XML data (pages 131–134) • Displayed your remote data in a Flex List (pages 135–136)
From the Library of Wow! eBook
LESSON 7
What You Will Learn In this lesson, you will: • Create an ActionScript class for use as a value object • Create ActionScript classes for a shopping cart • Add functionality to the ShoppingCart and ShoppingCartItem classes
Approximate Time This lesson takes approximately 1 hour and 15 minutes to complete.
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Lesson 7
Creating Classes Objects are the core of any object-oriented language. So far you have used objects provided for you by Adobe; however, to accomplish anything of even marginal complexity in Flex, you need to be comfortable creating your own. Objects are the realization of classes. Another way to state this is that a class is a blueprint for an object that will be created. In this lesson, you will first create several classes and then use them throughout the application.
The finished Product data structure built in ActionScript 3.0 and integrated into the application
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LESSON 7: Creating Classes
Building a Custom ActionScript Class As mentioned at the end of Lesson 2, “Getting Started,” this book does not aspire to teach object-oriented programming (OOP), but every Flex developer needs at least a working knowledge of OOP terminology. So if you are not familiar with terms like class, object, property, and method, now is a good time to take advantage of the hundreds, if not thousands, of OOP introductions around the web and in books. You have already been building custom ActionScript classes in this book but may not have been aware of it because Flex initially hides this fact from you. When you build an application in MXML, you are actually creating a new ActionScript class. Your MXML is combined with the ActionScript in the Script block, and a pure ActionScript class is created, which is then compiled into a SWF file for Flash Player. In the previous exercise, when you compiled FlexGrocer.mxml, a file named FlexGrocer-generated.as was created behind the scenes that contained the following code: public class FlexGrocer extends spark.components.Application
You extended the Application class when you built FlexGrocer.mxml and Checkout.mxml. The same is true for every application you create using Flex. Tip: If you wish to see the ActionScript created, you can add a compiler argument in Flash Builder . Navigate to Project > Properties > Flex Compiler > Additional compiler arguments, and add -keep-generated-actionscript to the end of the existing arguments . A folder named bin-debug/generated will be created automatically in your project, and many ActionScript files will be placed there . Your application files will be in the form Name-generated .as . Don’t forget to remove the compiler argument when you’ve finished exploring .
In the first exercise of this lesson, you will build a class directly in ActionScript, without relying on Flex to convert MXML into ActionScript. Ultimately this will give you much more granular control over your final code and encourage code reuse.
Building a Value Object Value objects, also called data transfer objects (DTOs), or just transfer objects, are objects intended to hold data. Unlike other objects you have used so far, such as Labels and DataGrids, value objects are free from any logic other than storing and retrieving their data. These objects are implemented as ActionScript classes.
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The name data transfer object comes from the fact that DTOs are often used for data transfer to the back end (server) of an application, often for permanent storage in a database. In this lesson, you will build a value object for a grocery product, along with objects for both a ShoppingCart and a ShoppingCartItem. Before you get started, the basics of building an ActionScript class need to be understood. A very simple class is shown here, and labeled for discussion: A B C D E
.
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Throughout the FlexGrocer application, you will need to display and manage typed data and send this data to different objects in the application. In this exercise, you will build a value object to hold information about a grocery product. 1 Open the FlexGrocer.mxml file that you used in the previous exercise. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson07/start folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Create a new ActionScript class file by choosing File > New > ActionScript class. Set the Package to valueObjects and the class Name to Product, and leave all other fields with the defaults. Click Finish to create the file. This process accomplished several things. First, it created a package named valueObjects, which you can now see in your Package Explorer. Next, it created a file named Product.as on your behalf. Finally, it populated that file with the required code for an ActionScript class. Within the code, the words package and class are both keywords used in defining this class. Remember that this class will be a blueprint for many objects that you will use later to describe each grocery product. 3 In the Product.as file you need to add a [Bindable] metadata tag on the line between the package definition and the class statement. package valueObjects { [Bindable] public class Product { public function Product() { } } }
The [Bindable] metadata tag, when specified before the line with the class keyword, means that every property in this class can be used in data binding (that is, it can be monitored for updates by various Flex controls). Instead of specifying the whole class as [Bindable], you can specify individual properties by locating the [Bindable] metadata tag over each property. For this application, you want every property in this class to be bindable. 4 Inside the Product class definition, add a public property with the name catID and the type Number. package valueObjects { [Bindable] public class Product {
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public var catID:Number; public function Product() { } } }
All properties of a class must be specified inside the class definition in ActionScript. 5 Create additional public properties with the names prodName (String), unitID (Number), cost (Number), listPrice (Number), description (String), isOrganic (Boolean), isLowFat (Boolean), and imageName (String). Your class should appear as follows: package valueObjects { [Bindable] public class Product { public var catID:Number; public var prodName:String; public var unitID:Number; public var cost:Number; public var listPrice:Number; public var description:String; public var isOrganic:Boolean; public var isLowFat:Boolean; public var imageName:String; public function Product() { } } }
You are creating a data structure to store inventory information for the grocery store. You have now created all the properties that will be used in the class. 6 When you created this class, Flash Builder created a default constructor on your behalf. Edit this constructor to specify the parameters that need to be provided when a new instance of the Product class is created. These parameters will match the names and types of the properties you defined in the last step. public function Product( catID:Number, prodName:String, unitID:Number, ➥cost:Number, listPrice:Number, description:String, isOrganic:Boolean, ➥isLowFat:Boolean, imageName:String ) { }
The constructor function is called when an object is created from a class. You create an object from a class by using the new keyword and passing the class arguments. In this case the parameter names match the property names of the class. This was done to keep things clear, but it is not necessary.
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NoTe: Two words are often used in discussions of methods: parameter and argument . They are often used interchangeably, but technically, functions are defined with parameters, and the values you pass are called arguments . So a function is defined to accept two parameters, but when you call it, you pass two arguments .
7 Inside the constructor, set each property of your object to the corresponding constructor parameter. When you are referring to the property of the class, you use the this keyword to avoid name collision (when the same name can refer to two separate variables). public function Product( catID:Number, prodName:String, unitID:Number, ➥cost:Number, listPrice:Number, description:String, isOrganic:Boolean, ➥isLowFat:Boolean, imageName:String ) { this.catID = catID; this.prodName = prodName; this.unitID = unitID; this.cost = cost; this.listPrice = listPrice; this.description = description; this.isOrganic = isOrganic; this.isLowFat = isLowFat; this.imageName = imageName; }
This code will set each property of the object to the corresponding argument passed to the constructor. The first line of the constructor reads: “Set the catID property of this object to the value that was passed to the catID parameter of the constructor.” Tip: You could name the constructor parameters differently from the properties (for example, categoryID instead of catID) . In that case, each property listed to the left of the equals sign
could have done without the this . prefix (for example, catID = categoryID;) . The prefix is added when you wish to be specific when referencing a property . The this prefix refers to the class itself and is implicit when there is no possibility of name collision .
8 Create a new method directly below the constructor function with the name toString() and the return type String. It will return the string [Product] and the name of the product. Your class should read as follows: package valueObjects { [Bindable] public class Product { public var catID:Number; public var prodName:String; public var unitID:Number; public var cost:Number; public var listPrice:Number; public var description:String;
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public var isOrganic:Boolean; public var isLowFat:Boolean; public var imageName:String; public function Product( catID:Number, prodName:String, unitID:Number, cost:Number, listPrice:Number, description:String, isOrganic:Boolean, isLowFat:Boolean, imageName:String ) { this.catID = catID; this.prodName = prodName; this.unitID = unitID; this.cost = cost; this.listPrice = listPrice; this.description = description; this.isOrganic = isOrganic; this.isLowFat = isLowFat; this.imageName = imageName; } public function toString():String { return “[Product]” + this.prodName; } } } toString() is a special method of objects in ActionScript. Whenever you use an instance of
your Product in a place where Flex needs to display a String, this method will be automatically invoked by Flash Player. A good example of this concept is the trace() method, which can be used to output data to the console. Running the code trace ( someProduct ) would call the toString() method of that Product instance and output the string it returns to the Console view. This can be very useful for debugging and displaying data structures. 9 Return to the FlexGrocer.mxml file and locate the Script block at the top of the page. Inside the Script block, declare a private variable named theProduct typed as a Product. Add a [Bindable] metadata tag above this single property. [Bindable] private var theProduct:Product;
If you used code completion, Flash Builder imported the Product class for you. If you did not, then add import valueObjects.Product; before continuing. All MXML files ultimately compile to an ActionScript class. You must follow the same conventions when creating an MXML class as when creating an ActionScript class. For example, you must import any classes that are not native to the ActionScript language, such as the Product class you have built, and you must declare any properties that you will use in your MXML class.
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10 Within the handleCreationComplete() method, but above the categoryService.send(); statement, create a new instance of the Product class and assign it to the theProduct property. When creating the new Product, you will need to pass a value for each constructor argument. For these arguments you will use the data from the tag named groceryInventory. Type the code as follows: theProduct = new Product( groceryInventory.catID, ➥groceryInventory.prodName, groceryInventory.unitID, ➥groceryInventory.cost, groceryInventory.listPrice, ➥groceryInventory.description, groceryInventory.isOrganic, ➥groceryInventory.isLowFat, groceryInventory.imageName );
Here you are instantiating a new object of that Product class you built. You are passing the data from the tag as constructor arguments. If you were to review the XML in the groceryInventory variable, you would note that it doesn’t presently have a node for catID and unitID. However, in this context, Flash Player will just interpret them as 0 (zero) for the Product value object. These values will be used extensively when you begin loading more complicated data from the server. NoTe: When you’re accessing properties from the groceryInventory XML, Flash Builder cannot help you with code completion or even compile-time checking . The nodes inside the XML aren’t checked, hence the reason Flash Builder does not complain when you type groceryInventory.catID even though it is not present in the XML . This means it is extremely easy to make a typo that can be difficult to debug . For now check your code carefully, but as we continue to use strongly typed objects, you will see how Flash Builder can help and why typed objects can make debugging easier .
11 On the next line, add a trace() statement and trace the property theProduct out to the console. This statement will automatically execute the toString() method of your object and output the results. Your finished method should look like the following: private function handleCreationComplete(event:FlexEvent):void { theProduct = new Product( groceryInventory.catID, ➥groceryInventory.prodName, groceryInventory.unitID, ➥groceryInventory.cost, groceryInventory.listPrice, ➥groceryInventory.description, groceryInventory.isOrganic, groceryInventory.isLowFat, groceryInventory.imageName ); trace( theProduct ); categoryService.send(); }
12 Save and debug the application.
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You should see [Product]Milk in the Console view, which indicates that you have created a Product value object successfully.
Building a Method to Create an Object As you just did in the previous exercise, you can instantiate an instance of the Product class by passing values as arguments to the constructor. In this exercise, you will build a method that will accept any type of object that contains all the properties and values needed for a product, and return an instance of the Product class populated with this data. This type of method is often referred to as a factory method, as its job is creating other objects. Note that for this method to function correctly, the object passed to the method must contain property names that correspond exactly to the names you will hard-code into this method. 1 Be sure the Product class in the valueObjects package is open. Locate the toString() method. Immediately after this method, add the skeleton of a new public static method called buildProduct(). Be sure that the return type of the method is set to Product, and that it accepts a parameter named o typed as Object, as shown: public static function buildProduct( o:Object ):Product { }
A static method is a method that belongs to a class, not to an instance. The methods you have worked with so far are called instance methods; they can be used only with instantiated objects. Consider for a moment your toString() method. That method uses productName to display the name of the product represented by that object. If you have n product objects (where n is any number of product objects), each should display a different name when the toString() method is called. Since this method uses data from the object, it is logical that the object must exist before the method can be called.
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Conversely, you may have a method that doesn’t need (or care) about any of the data inside a specific instance. In fact, it could just be a utility method that does work independent of any particular instance. This is called a static method. Static methods are often used for utilities such as the buildProduct() method. You will be able to call this method without creating an instance of the Product first. Used appropriately, static methods can increase the legibility and usefulness of your objects. To reference a static method with the name buildSomething() from the Product class, you would use the code Product.buildSomething(), which uses the class name before the method, as opposed to the instance. 2 Inside the buildProduct() method, create a new local variable named p of type Product. var p:Product;
3 Below the local variable declaration, instantiate an instance of the Product class assigning it to p using the new keyword. Pass the catID, prodName, unitID, cost, listPrice, description, isOrganic, isLowFat, and imageName properties of the object o as arguments to the constructor. The isOrganic and isLowFat variables will be compared against the String ‘true’ before being passed as the code below demonstrates: p = new Product( o.catID, o.prodName, o.unitID, o.cost, ➥o.listPrice, o.description, ( o.isOrganic == 'true' ), ➥( o.isLowFat == 'true' ), o.imageName );
Remember that the data used here is retrieved from the tag. When data is retrieved this way, all the data is XML: The true and false values for these fields are just treated as a type of String. Comparing isOrganic and isLowFat to the String 'true' will return a Boolean value, either a true of a false. In this way, you are converting the value contained in the XML to the Boolean value that the newly created object is expecting for these properties. Converting and the related concept of casting allow you to treat a variable of a given property as another type. 4 Return the object you just created by using the return keyword with the name of the object, p. Your final buildProduct() method should appear as follows: public static function buildProduct( o:Object ):Product { var p:Product; p = new Product( o.catID, o.prodName, o.unitID, o.cost, ➥o.listPrice, o.description, ( o.isOrganic == 'true' ), ➥( o.isLowFat == 'true' ), o.imageName ); return p; }
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This method will create and return a new Product value object and populate it with data from the object passed as an argument. 5 Save the Product.as file. The class file is saved with the new method. No errors should appear in the Problems view. 6 Return to FlexGrocer.mxml. In the handleCreationComplete() method, remove the code that builds theProduct and replace it with code that uses the static method to build theProduct. Remember to remove the new keyword. theProduct = Product.buildProduct( groceryInventory );
This code calls the static method that builds an instance of the Product class, which returns a strongly typed Product value object from any type of object that has correspondingly named properties. 7 Locate the VGroup container, which, in the expanded state, displays the product description (whether the product is organic, and whether it is low fat). Change the text property of the tag to reference the description property of the theProduct object you created in the handleCreationComplete() method. Also, add a visible property to both labels, and bind each to the appropriate theProduct object properties, as shown in the following code. Tip: Remember, because Product is now an imported class, you can get code hinting for both the class name and its properties . When you are in the braces creating the binding, press Ctrl-Spacebar to get help for inserting the Product instance, theProduct . Then, after you enter the period, you will get the properties listed .
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You are now referencing the value object you created. You also just gained one more benefit: Flash Builder is now helping you debug. If you were to make a typo—for example, if you were to type theProduct.isLowerFat—Flash Builder would alert you to the error when you saved. Now that Flash Builder knows the types of the objects you are using, it can verify that you are accessing properties that exist. What might have taken you a few minutes to reread, check, and perhaps even debug, Flash Builder found in moments. Over the course of a project, that becomes days and weeks of time. 8 Save and debug the application. You should see that the trace() method performs just as before, and the correct data should still appear when you roll over the image.
Building Shopping Cart Classes In this exercise, you will build a new class called ShoppingCartItem. This class will contain an item added to a shopping cart that you will also build shortly. The new class will keep track of a product added and its quantity. You will also build a method that calculates the subtotal for that item. You will then build the skeleton for a ShoppingCart class that will handle all the logic for the shopping cart, including adding items to the cart. 1 Create a new ActionScript class file by choosing File > New > ActionScript class. Set the Package to cart, which automatically adds this class to a folder named cart inside your project. Enter ShoppingCartItem as the Name and leave all other fields with default values. In this class, you will calculate the quantity of each unique item as well as the subtotal.
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2 Within the class definition, define a public property with the name product and the type Product, as shown: package cart { public class ShoppingCartItem { public var product:Product; public function ShoppingCartItem() { } } }
The product is the most important piece of data in the ShoppingCartItem. 3 Define a public property with the name quantity of the type uint, as shown: package cart { public class ShoppingCartItem { public var product:Product; public var quantity:uint; public function ShoppingCartItem() { } } }
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The data type uint means unsigned integer, which is a nonfractional, non-negative number (0, 1, 2, 3, …). The quantity of an item added to the shopping cart will be either zero or a positive number, so uint is the perfect data type. 4 Define a public property with the name subtotal and the data type Number, as shown: package cart { public class ShoppingCartItem { public var product:Product; public var quantity:uint; public var subtotal:Number; public function ShoppingCartItem() { } } }
Each time a user adds an item to the shopping cart, you will want the subtotal for that item to be updated. In this case, you are using Number as the data type. As the product’s price is not likely to be an integer, the Number class allows for fractional numbers. Eventually, you will display this data in a visual control. 5 Edit the signature of the constructor of this class and specify the parameters that will be passed to this function. These parameters will include product typed as a Product and quantity typed as a uint. You will provide a default value of 1 for the quantity. If the developer calling this method does not provide a quantity, you will assume 1. public function ShoppingCartItem( product:Product, quantity:uint=1 ) { }
Remember that a constructor function must be public and that it never specifies a return type. 6 In the constructor, assign the object’s properties to the values passed into the constructor’s parameters. The names used are the same, so prefix the properties on the left side of the equal sign with this. public function ShoppingCartItem( product:Product, quantity:uint=1 ){ this.product = product; this.quantity = quantity; }
Remember that the constructor is called every time an object is created from a class. The constructor will set the properties that are passed in—in this case, an instance of the Product class, and the quantity, which is set to 1 as a default. This object will be used only when an item is added to the shopping cart, so a default quantity of 1 seems reasonable.
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7 Create a public method with the name calculateSubtotal() that will calculate the subtotal of each item by multiplying the listPrice of the product by the quantity, as follows: public function calculateSubtotal():void{ this.subtotal = product.listPrice * quantity; }
When the user adds items to the shopping cart, you need to perform calculations so that the subtotal can be updated. Eventually, you also need to check whether the item has already been added to the cart; if so, you will update the quantity. You will learn how to do this in the next lesson. 8 Call the calculateSubtotal() method on the last line of the constructor. This will ensure that the subtotal is correct as soon as the object is created. public function ShoppingCartItem( product:Product, quantity:uint=1 ) { this.product = product; this.quantity = quantity; calculateSubtotal(); }
9 Create a public method with the name toString() that will return a nicely formatted string with the product’s name and quantity. The returned string will read [ShoppingCartItem], followed by a space, the product’s name, a colon, and finally the quantity of that product in this ShoppingCartItem. public function toString():String { return “[ShoppingCartItem] “ + product.prodName + “:” + quantity; }
As you learned previously, toString() methods are automatically called when Flash Player needs to represent this object as a String, such as when you use it in a trace() statement. This will provide you a lot of valuable debugging information. 10 You will now create another new class. Start by choosing File > New > ActionScript class. Set the package to cart. Name the class ShoppingCart and leave all other fields with default values. Your new class will be the actual shopping cart, filled with ShoppingCartItem objects. This class will handle the manipulation of the data in the shopping cart. You have already created the visual look and feel of the shopping cart, and you will place all your business logic in this new class. This business logic includes work that must occur when adding an item to the cart, deleting an item from the cart, updating an item in the cart, and so on.
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11 Create the skeleton of a public addItem() method, which returns void. The method will accept a parameter named item, of type ShoppingCartItem. In the method, add a trace statement that will trace the item added to the cart: package cart { public class ShoppingCart { public function ShoppingCart() { } public function addItem( item:ShoppingCartItem ):void { trace( item ); } } }
This is the method in which you will add a new item to the shopping cart. You will add more business logic to this method later. For now, you will just trace the item added to the cart. Remember that the toString() method you wrote earlier is called automatically whenever an instance of the ShoppingCartItem class is traced. 12 Open FlexGrocer.mxml in Flash Builder and locate the Script block. Just below the import statement for the Product value object, import the ShoppingCartItem and ShoppingCart classes from the cart folder, as shown: import cart.ShoppingCartItem; import cart.ShoppingCart;
To use a class in a different package, your application needs an import statement that references the location or package in which the class is located. 13 Just below the import statements, instantiate a public instance of the ShoppingCart class, name the instance shoppingCart, and add a [Bindable] metadata tag, as follows: [Bindable] public var shoppingCart:ShoppingCart = new ShoppingCart();
Pay attention to the differences in case here. Variables, such as shoppingCart, usually start with a lowercase letter. Classes, such as ShoppingCart, start with an uppercase letter. When the user clicks the AddToCart button, you want to call the addItem() method of the ShoppingCart class you just created. You will pass the addItem() method an instance of the ShoppingCartItem class. By instantiating the class here, you ensure that you have access to it throughout the application.
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14 Locate the handleViewCartClick() method in the block. Immediately after this method, add a new private function with the name addToCart() that returns void. Have the method accept a parameter named product typed as Product, as shown: private function addToCart(product:Product):void { }
This method will be called when the user clicks the AddToCart button, and you will pass an instance of the Product value object. As you do not intend anyone to call this method from outside this MXML class, you can use the private identifier. Using the keyword private here prevents others from calling this method unexpectedly. 15 Inside the addToCart() method, create a new instance of the ShoppingCartItem class with the name sci and pass the product parameter as an argument to the constructor. private function addToCart( product:Product ):void { var sci:ShoppingCartItem = new ShoppingCartItem( product ); }
Notice that you passed the product but you did not pass the second parameter of the constructor (quantity). This is okay, as you provided a default value for the quantity when creating this class. 16 On the next line of the addToCart() method, call the addItem() method of the shoppingCart instance of the ShoppingCart class. Be sure to pass the sci object you just created to the method, as follows: private function addToCart( product:Product ):void { var sci:ShoppingCartItem = new ShoppingCartItem( product ); shoppingCart.addItem( sci ); }
This code will call the addItem() method of the ShoppingCart class you built earlier. In the next sections, you will learn how to loop through the data structure to see whether the item is added. For now, this method simply traces the name of the product added to the cart. 17 Find the AddToCart button and add a handler for the click event that calls the addToCart() method, passing an instance of theProduct:
Remember, the addToCart() method creates an instance of the ShoppingCartItem class and then passes that object to the shopping cart.
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18 Save and debug the application. Each time you click the AddToCart button, you should see [ShoppingCartItem] Milk:1 appear in the Console view.
Manipulating Shopping Cart Data The next several exercises all deal with manipulating data in the shopping cart. You will work extensively with the Array class. In Lesson 8, “Using Data Binding and Collections,” you will add even more functionality to this class and allow it work with Flex controls to update the display dynamically.
Adding Items to the Cart In this exercise you will write the code to add items to the shopping cart. 1 Open the ShoppingCart.as file from the cart package that you used in the previous exercise. Alternatively, if you didn’t complete the previous exercise or your code is not functioning properly, you can import the FlexGrocer-PreCartData.fxp project from the Lesson07/ intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 On the line after the class definition, define a public property with the name items, typed as an Array and set equal to a new Array instance. package cart { public class ShoppingCart { public var items:Array = new Array();
This instantiates an Array object and assigns it to the items property. You will use this Array to track all the objects in the shopping cart.
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3 Define a public property with the name total, typed as a Number. It will hold the total price of the items in this class. When the class is first instantiated, there won’t be any items, so set the value to 0 as shown: public var total:Number=0;
Anytime a user adds an item to the cart, you will update this property with the price of the item. This will enable you to track the total cost of the end user’s order. 4 Locate the addItem() method of the ShoppingCart class and remove the trace statement. Use the push() method of the Array class to add ShoppingCartItem to the items array. The push() method of the Array adds an element to the end of the array, after any existing items. public function addItem( item:ShoppingCartItem ):void { items.push( item ); }
In the previous exercise, you built a ShoppingCartItem class to hold data associated with items in a shopping cart. This class has properties to hold the product (an instance of the Product class), the quantity (an unsigned integer), and the subtotal (a number derived by multiplying the quantity by the price). When the user clicks the AddToCart button, you pass a ShoppingCartItem to this method and place it in the Array using addItem(). 5 Create a public method with the name toString() that will return a nicely formatted string representing the items in the shopping cart. The returned string will be [ShoppingCart, followed by a space, a dollar sign, the closing right bracket, another space and the items array as shown: public function toString():String { return “[ShoppingCart $” + total + “] “ + items; }
As you learned previously, toString() methods are called automatically when Flash Player needs to represent this object as a String, such as when you use it in a trace() statement. You added toString() methods to several of your objects already. When this statement executes, it is going to display [ShoppingCart $0] and then the array. The Array also has a toString() method, so when you trace this array, it will actually call the toString() method on each item in the array. 6 Switch back to FlexGrocer.mxml and locate the addToCart() method. To the last line of this method add a trace() statement that traces the shoppingCart. private function addToCart( product:Product ):void { var sci:ShoppingCartItem = new ShoppingCartItem( product ); shoppingCart.addItem( sci ); trace( shoppingCart ); }
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Each time you click the AddToCart button, the entire shopping cart will be traced to the console. 7 Save and debug the application. Each time you click the AddToCart button, you will see another line appear in the Console view. For example, clicking it three times will yield: [ShoppingCart $0] [ShoppingCartItem] Milk:1 [ShoppingCart $0] [ShoppingCartItem] Milk:1,[ShoppingCartItem] Milk:1 [ShoppingCart $0] [ShoppingCartItem] Milk:1, [ShoppingCartItem] Milk:1,[ShoppingCartItem] Milk:1
The number of items grows each time another Milk product is added to the cart. However, this uncovers an error in your ShoppingCart. If you add Milk a second time, you likely mean to increase the quantity by 1, not actually add a new item. You will address this over the next few exercises.
Adding an Item or Updating the Quantity The code you are about to write conditionally places a new item, or updates an existing item, in the shopping cart. It is not difficult line by line, but the logic involved can seem complex at first. To be sure you understand the big picture before you try to implement the details, let’s walk through the logic required for the implementation. 1. The user clicks a button to add an item to the shopping cart. 2. The addItem() method is called. 3. The cart is checked to see if the item already exists. • If the item does not exist, it is added. • If the item does exist, you find and update the existing item. 4. The cart’s total is updated.
Conditionally Adding a ShoppingCartItem A key point in the logic of this exercise is determining whether a newly added ShoppingCartItem is already in the existing array of ShoppingCartItems. In this section, you will simply loop through the array looking for the correct item. In the next lesson, you will learn to use collections and cursors to make this faster and more elegant.
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Finding an Item in the Cart The desired behavior is to have only new items added to the cart. If an item is clicked more than once, it should update the quantity of the item. You will need to write a method that can check whether an existing product is in the cart already. 1 In the ShoppingCart.as file, add a new private function named getItemInCart(). This method will accept a single parameter named item of type ShoppingCartItem. The method will return a ShoppingCartItem. private function getItemInCart( item:ShoppingCartItem ):ShoppingCartItem { }
This method will accept a ShoppingCartItem and then look through all existing items to see if the represented product already exists in the cart. 2 On the first line of the getItemInCart() function, declare a new local variable named existingItem of type ShoppingCartItem . private function getItemInCart( item:ShoppingCartItem ):ShoppingCartItem { var existingItem:ShoppingCartItem; }
3 Directly below that variable declaration, create a for loop. Loops are blocks of code that are executed repeatedly using a series of values. In this loop, declare a variable i of type uint and loop from 0 to less than the length of the items array. In ActionScript, this loop appears like the following code: private function getItemInCart( item:ShoppingCartItem ):ShoppingCartItem { var existingItem:ShoppingCartItem; for ( var i:uint=0; i
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7 Cut the states block from the main application, and paste it in the component, between the closing tag and the starting tag.
The reality is that the order of where you place the states tag is unimportant, as long as you do not place it as the child of something other than the root node. 8 Cut the addToCart() and removeCart() methods from the main application, and paste them into your component in the block. Use the code-completion feature to import the Product and ShoppingCartItem classes. Remove the trace ( shoppingCart ); statement from your addToCart() method. It is no longer needed. private function addToCart( product:Product ):void { var sci:ShoppingCartItem = new ShoppingCartItem( product ); shoppingCart.addItem( sci ); } private function removeFromCart( product:Product ):void { var sci:ShoppingCartItem = new ShoppingCartItem( product ); shoppingCart.removeItem( sci ); }
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At this point, the full Script block should read as:
These methods are called from some of the MXML tags you moved into the ShoppingView class, so they need to be moved into that component. 9 Copy the handleViewCartClick() method from the main application, and paste it into your component’s block. private function handleViewCartClick( event:MouseEvent ):void { this.currentState = “cartView”; }
At this point, the full Script block should read as follows:
These methods are called from some of the MXML tags you moved into the ShoppingView class, so they need to be moved into that component. 10 Save the file. You have created your first MXML component. Now that the component is built, you will instantiate the new component from the main application. 11 Return to the FlexGrocer.mxml file and find the Label that shows the copyright mark. Just after that tag, type in MXML Component. The name should be ProductItem, and the base component should be Group with a Horizontal layout. Remove the width and height values, and then click Finish.
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3 Add an tag pair just after the layout declaration. Much as with the last component, the Script block will be used to add properties and methods for your new component. 4 Add a bindable public property named product, with a data type of the Product class, to the Script block. Use code-completion to import the Product class, or manually add an import statement for valueObjects.Product. Add another public variable, called shoppingCart, with a data type of ShoppingCart. This will allow you to pass a specific product to each instance of this component, and pass a reference to the shoppingCart to each instance as well. Remember to either use the code-completion functionality when specifying the ShoppingCart class, or to manually add an import for cart.ShoppingCart. import cart.ShoppingCart; import valueObjects.Product; [Bindable] public var product:Product; public var shoppingCart:ShoppingCart;
5 Cut the addToCart() and removeFromCart() methods from the ShoppingView component, and paste them into the Script block of the ProductItem component. You will need to make sure the ShoppingCartItem class is imported as well. import cart.ShoppingCart; import cart.ShoppingCartItem; import valueObjects.Product; [Bindable] public var product:Product; public var shoppingCart:ShoppingCart; private function addToCart( product:Product ):void { var sci:ShoppingCartItem = new ShoppingCartItem( product ); shoppingCart.addItem( sci ); } private function removeFromCart( product:Product ):void { var sci:ShoppingCartItem = new ShoppingCartItem( product ); shoppingCart.removeItem( sci ); }
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6 In ShoppingView.mxml, cut the VGroup with the id of products, and the one that follows it, which is included in the expanded state, and paste them after the section of ProductItem
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7 Copy the states block from ShoppingView.mxml, and paste it into ProductItem.mxml just after the layout tag pair. In ShoppingView, remove the state definition for expanded. In ProductItem, remove the state definition for cartView. Your ShoppingView states block should read like this:
Your ProductItem states block should read like this:
Now both components have a base state (State1), and each has another state specific for it. ShoppingView has the cartView state, which it can use to show the details of a shopping cart, and ProductItem has an expanded state, which shows expanded product details. 8 As ProductItem no longer has a cartView state, you need to remove the attributes of the first VGroup that explicitly set width, height, and visible properties for the cartView state. While removing the attributes, also remove the normal width and height attributes as well, as those will no longer be needed, either.
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9 Change the reference for the image source from the current embedded image “@Embed(‘assets/dairy_milk.jpg’)” and instead dynamically load the image from the assets directory, using the image name of the product.
Your component can now show the appropriate image for any product passed to it, rather than always showing milk. 10 In the Label just before the image, change the text=”Milk” to text=”{product.prodName}” to dynamically display the product name.
Your component can now show the correct name for whichever product it has. 11 In the Label just after the image, change the text=”$1.99” to text=”${product.listPrice}” to dynamically display the product price.
Your component can now show the correct price for whichever product it has. 12 In the click handlers for both the Add To Cart and Remove From Cart buttons, change the argument passed to the function from groceryInventory.getItemAt( 0 ) as Product to product.
Since your component is no longer dealing with the entire groceryInventory collection, but instead with an individual product, the reference to the product for this component is now greatly simplified. When you create an instance of this component from the ShoppingView component, you will pass just one specific product to each instance.
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13 For the RichText control and two labels in the VGroup shown in the expanded view, change the reference in the binding from groceryInventory.getItemAt( 0 ) as Product to just product.
Your final code for the ProductItem component should read like this:
Next, you will need to create one or more instances of this component from the ShoppingView. 14 Switch back to ShoppingView.mxml. After the tag pair, but before the cartGroup VGroup, create a new VGroup with a width and a height of 100% . Inside this group, create an instance of ProductItem. If you begin typing it and use code-completion, as you did when you created the instance of ShoppingView in the previous lesson, the import statement for the components package will be automatically added.
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15 Give the new ProductItem instance an id=”product1", specify a width and a height of 100%, bind a reference of the local shoppingCart into the shoppingCart property of the new component, and bind groceryInventory.getItemAt(0) as Product to its product property.
16 Save all the files and run the application.
Your application is now displaying the first item from the groceryInventory, which is buffalo meat, rather than the milk you have been used to seeing in the application thus far. But wait, there’s more. Since you now have a component that can easily show individual products, you can show several at once. 17 Switch back to ShoppingView.mxml. Copy the tag that creates an instance of ProductItem, and paste it twice more, just below the current one. Change the id of the new ones to be product2 and product3. Also change the binding to the product property to use item 1 and item 2, while the original is getting item 0.
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One bug still needs to be fixed. If you click the View Cart button, the products are still being shown, rather than being hidden. You will fix that in the next step.
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18 In the VGroup that contains the ProductItems, specify values for the width and the height of the cartView state to be 0, and the visible property to be false.
The application should now be able to switch between the various states of the ProductItem and ShoppingView components correctly, while now displaying three products instead of just one.
Creating Components to Manage Loading the Data In the first exercise, you refactored part of the application without adding any functionality. In the second exercise, you added functionality (showing multiple products) while building another component. This exercise is akin to the first, in which you are refactoring the application without adding any visible functionality for the user. Right now, the main application file is a bit cluttered by the instantiation and event handlers for the two HTTPServices. In this exercise, you are going to create ActionScript classes for these services, which will contain the HTTPService components as well as result and fault event handlers, and will expose the data loaded through public bindable properties. The new class will provide certain types of data to all the applications when they need it. This data manager component will be different from other components you’ve built in this lesson in that it will not have any representation that a user will see. Such a component is referred to as a non-visual component. 1 Create a new services package in the FlexGrocer project. Alternatively, if you didn’t complete the previous exercise or your code is not functioning properly, you can import the FlexGrocer-PreData.fxp project from the Lesson09/intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. Because the new components are neither a value object nor a view, a new package is needed to continue organizing your components by function. 2 Right-click the services folder and then choose New ActionScript Class. In the New ActionScript Class dialog box, set the name as CategoryService and set the superclass as a mx.rpc.http.mxml.HTTPService; leave the rest of the defaults, then click Finish.
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As you want a class that provides all the functionality of HTTPService, but has some additional methods and properties, HTTPService is the most logical choice for a base class. 3 After the line declaring public class CategoryService but before the constructor, add a bindable public variable categories:XMLListCollection. import mx.collections.XMLListCollection; import mx.rpc.events.ResultEvent; import mx.rpc.http.mxml.HTTPService; public class CategoryService extends HTTPService { [Bindable] public var categories:XMLListCollection; public function CategoryService(rootURL:String= ➥null, destination:String=null)
This categories property will determine how other classes interact with the data loaded by the service. Don’t forget to use the code-hinting feature, or to manually import the XMLListCollection class.
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4 In the constructor, after the call to super(), set the resultFormat property of your class equal to e4x and the url property to http://www.flexgrocer.com/category.xml. public function CategoryService(rootURL:String=null, destination:String=null) { super(rootURL, destination); this.resultFormat=”e4x”; this.url=”http://www.flexgrocer.com/category.xml”; }
Take a look at the constructor here. The first line inside the function definition (which was automatically added by the new-class wizard), passes the rootURL and destination arguments to the constructor of the superclass. This way, it is not necessary to duplicate the logic found in the superclass’s constructor. The two lines you added are setting the resultFormat and url properties of the HTTPService class, as you learned in previous lessons. 5 Open FlexGrocer.mxml, cut the handleCategoryResult() method, and paste it into the new CategoryService class, after the constructor. private function handleCategoryResult( event:ResultEvent ):void { categories = new XMLListCollection( event.result.category ); }
As with each new class you introduce, make sure the other classes you are using get imported. In CategoryService, you need to ensure that the ResultEvent class gets imported, either by typing in the import statement for mx.rpc.events.ResultEvent manually, or by using the code-completion feature. This method will populate the categories property with the results from the service call. 6 In the constructor, add an event listener for the result event. Set handleCategoryResult as the handler for that event. addEventListener(ResultEvent.RESULT, handleCategoryResult);
The addEventListener() method allows you to specify an event to listen for (in this case it’s the event result and a method that will be used as the event handler). 7 Save CategoryService. Switch to the FlexGrocer.mxml file. Your service class is now complete. All that remains is to use it in the application. The completed CategoryService class should read like this: package services { import mx.collections.XMLListCollection; import mx.rpc.events.ResultEvent;
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import mx.rpc.http.mxml.HTTPService; public class CategoryService extends HTTPService { [Bindable] public var categories:XMLListCollection; public function CategoryService(rootURL:String=null, ➥destination:String=null) { super(rootURL, destination); this.resultFormat="e4x"; this.url="http://www.flexgrocer.com/category.xml"; addEventListener(ResultEvent.RESULT, handleCategoryResult); } private function handleCategoryResult( event:ResultEvent ):void { categories = new XMLListCollection( event.result.category ); } } }
8 In the block of FlexGrocer.mxml, delete the tag with the id of categoryService.
9 In its place, create an instance of the CategoryService class. Give this new instance an id of categoryService. As with the previous components you instantiate, if you use the code-hinting features, the namespace will be automatically added for you.
You now have your new component being used in place of (and in fact with the same id as) the previous HTTPService. Since the id is the same, the existing call to categoryService.send() in the handleCreationComplete() method does not need to change. 10 Remove the bindable private categories property. You will no longer need this, as the categories are now available from the categoryService instance directly.
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11 Find the List class created inside the controlBarContent. Change the dataProvider from categories to categoryService.categories.
The FlexGrocer application is now using your new CategoryService class, instead of having the service properties and handlers all coded into the main application. 12 Save all your files and run the FlexGrocer application. It should now behave as it always did, with the categories loaded into the horizontal list.
Next, you will create a service class similar to CategoryService to load and manage the products, and you’ll remove that logic from the main application. 13 Close all your open files. Right-click the services folder and then choose New ActionScript Class. In the New ActionScript Class dialog box, set the Name as ProductService and set the Superclass as mx.rpc.http.mxml.HTTPService; leave the rest of the defaults, then click Finish.
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14 After the line declaring public class ProductService but before the constructor, add a bindable public variable products:ArrayCollection. [Bindable] public var products:ArrayCollection;
This products property will determine how other classes interact with the data loaded by the service. Don’t forget to use the code-completion feature, or to manually import the ArrayCollection class. 15 In the constructor, after the call to super(), set the resultFormat property of your class equal to e4x and the url property to http://www.flexgrocer.com/categorizedProducts.xml. public function ProductService(rootURL:String=null, destination:String=null) { super(rootURL, destination); this.resultFormat=”e4x”; this.url=”http://www.flexgrocer.com/categorizedProducts.xml”; }
The constructor is just like the one for the CategoryService, with a different url property.
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16 Open FlexGrocer.mxml, cut the handleProductResult() method, and paste it into the new ProductService class, after the constructor. Change the final line so that it populates the products property rather than the groceryInventory property. Change the local products variable to be productArray. private function handleProductResult( event:ResultEvent ):void { var productArray:Array = new Array(); var resultData:XMLList = event.result..product; for each (var p:XML in resultData) { var product:Product = Product.buildProductFromAttributes( p ); productArray.push( product ); } products = new ArrayCollection( productArray ); }
As with each new class you introduce, make sure you import the newly referenced classes (ResultEvent and Product), either by typing in the import, or by using the code-completion feature. This method will parse the results of the service call into Product instances and populate the products property with them. 17 In the constructor, add an event listener for the result event. Set handleProductResult() method as the handler for that event. addEventListener(ResultEvent.RESULT, handleProductResult);
Just as with the CategoryService class, you will want to listen for the result event, and pass the results on to a handler method. The final ProductService class should read like this: package services { import mx.collections.ArrayCollection; import mx.rpc.events.ResultEvent; import mx.rpc.http.mxml.HTTPService; import valueObjects.Product; public class ProductService extends HTTPService { [Bindable] public var products:ArrayCollection; public function ProductService(rootURL:String=null, ➥destination:String=null) { super(rootURL, destination);
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addEventListener(ResultEvent.RESULT, ➥handleProductResult); this.resultFormat="e4x"; this.url="http://www.flexgrocer.com/categorizedProducts.xml"; } private function handleProductResult( ➥event:ResultEvent ):void { var productArray:Array = new Array(); var resultData:XMLList = ➥event.result..product; for each (var p:XML in resultData) { var product:Product = ➥Product.buildProductFromAttributes( p ); productArray.push( product ); } products = new ArrayCollection( productArray ); } } }
18 Save ProductService. Switch to the FlexGrocer.mxml file. Your service class is now complete. All that remains is to use it in the application. In the block of FlexGrocer.mxml, delete the tag with the id of productService. In its place, create an instance of the ProductService class. Give this new instance an id of productService. As with the previous components you instantiate, if you use the code-hinting feature, the namespace will be automatically added for you.
Since the id is the same, the existing call to productService.send() in the handleCreationComplete() method does not need to change. 19 Remove the bindable private groceryInventory property and the Bindable public shoppingCart property. You will no longer need these, as the products are now available from the productService instance’s products property and the ShoppingCart is now defined in the ShoppingView. 20 With the exception of mx.events.FlexEvent you can now remove all of the imports from this file. They are no longer needed as the functionality has been moved to components.
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21 Find the ShoppingView class. Change groceryInventory=”{groceryInventory}” to be groceryInventory=”{productService.products}”
Your refactoring of the FlexGrocer application into components is now complete. 22 Save all your files and run the FlexGrocer application. It should now behave as it always did, but now in an easier-to-maintain fashion. Your refactored FlexGrocer file should now read like this:
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What You Have Learned In this lesson, you have: • Gained a theoretical understanding of why components should be used and how they fit into a simple implementation of MVC architecture (pages 204–209) • Built a component that moved the visual elements from a main application page to the component and then instantiated the component in the main application page (pages 210–226) • Created non-visual components that provide category and product information to the applications (pages 226–235)
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LESSON 10
What You Will Learn In this lesson, you will: • Populate a List control with a dataset • Populate a DataGroup with a dataset and display the information using a renderer • Create an MXML component to be used as a renderer • Use the Generate Getter/Setter wizard • Learn about virtualization • Respond to a user’s selection from a list
Approximate Time This lesson takes approximately 2 hours to complete.
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Lesson 10
Using DataGroups and Lists In this lesson, you will expand your skill in working with datasets. A dataset is really nothing but several data elements consolidated in a single object, like an Array, XMLList, ArrayCollection, or XMLListCollection. Up to this point, you have learned a few ways to display, manipulate, or loop over these datasets. In this chapter, you will learn about Flex components that automatically create a visual element for each item in a dataset.
A dataset is used with a horizontally laid-out List to display grocery categories and with a DataGroup to display the grocery items from that category.
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In this lesson, you will learn about Lists and DataGroups. Both List and DataGroup instances can create a visual element for each item in its dataset (which is set to the DataGroup’s dataProvider property). What is shown for each element will depend on the itemRenderer being used. You will learn about itemRenderers in this lesson as well. The List class, much like the DataGroup class, has a dataset in its dataProvider and will visually represent each item using its itemRenderer. Lists add another piece of functionality, in that they manage the user’s selection of items from the list and provide an API for determining which item(s) if any, are selected. In the course of this lesson, you will rework the ShoppingView component. Instead of having a hard-coded set of ProductItems as children, the component uses a DataGroup to dynamically create one ProductItem for each element in the groceryInventory ArrayCollection. In this process, you will rework the ProductItem class to be an itemRenderer. You will also finish building out the functionality of the List displaying categories at the top of the application and will learn how to make the ShoppingView change the contents of its groceryInventory property when the user selects one of the categories.
Using Lists In the application, you have already used two List instances, one with a horizontal layout to display the categories across the top of the application, and the other to display the items in the shopping cart. From your use of these two Lists, you know that the List class is provided with a dataset via dataProvider property (one list is using a XMLListCollection, and the other an ArrayCollection), and the list will display one item for each element in its dataProvider. In Lesson 6, “Using Remote XML Data,” you used a list to display the categories in the control bar. In that list, you specified a labelField to indicate which property the list should display. Using the labelField property is a very effective way of specifying which property of an object will be shown for each item of the list; however, it is limited in that it can display only text. If you want to format the data, or concatenate multiple properties, you will need to use a labelFunction.
Using a labelFunction with a List A labelFunction is a function that is used to determine the text to be rendered for each item in a List. This is done with the labelFunction property. The function will accept an Object as a parameter (if you are using strongly typed objects, you can specify the actual data type instead of the generic). This parameter represents the data to be shown for each item displayed by the List. The following code shows an example of a labelFunction, which displays the category of an item with its name and cost.
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If this application were saved and run, it would appear like this:
Each object from the dp ArrayCollection is passed into the labelFunction() before it is rendered, and whatever value is returned from that function is what will be shown. In this case, you are displaying the category name, the item’s name, and then its cost. NoTe: Although the multiDisplay function accepts parameters (private
function
multiDisplay(item:Object):String), you only pass a reference to the function to the List’s labelFunction property (labelFunction=”multiDisplay”) . Flex will automatically call the
function with the correct arguments as it renders each item from the dataProvider .
In this next exercise, you will use a labelFunction to format the data rendered in the shopping cart list. 1 Open the ShoppingView class. 2 Create a private function named renderProductName(), which accepts a ShoppingCartItem as a parameter and returns a String. private function renderProductName( item:ShoppingCartItem ):String { }
Make sure you either add the import for ShoppingCartItem manually, or use codecompletion to auto-import it. 3 As the first line of the function, create a local variable, data typed as a Product, which is equal to the product property of the parameter to the function. Then, construct and return a string that concatenates parentheses around the item.quantity, followed by product.prodName, a dollar sign, and then the item’s subtotal. private function renderProductName( item:ShoppingCartItem ):String { var product:Product = item.product; return ‘(‘ + item.quantity + ‘) ‘ + product.prodName + ‘ $’ + item.subtotal; }
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Make sure you either add the import for Product manually, or use code-completion to auto-import it. 4 Find the list in the cartGroup, and instruct it to use the renderProductName labelFunction.
5 Save and run the application. Notice how the items are formatted in the cart as you add products.
Using DataGroups In previous lessons, you learned that the Flex 4 framework includes a container class named Group, which can be used to contain any arbitrary visual elements as children and apply a layout to them. A DataGroup follows the same concept, but rather than requiring the number of children to be explicitly defined, it allows you to pass a dataset, and it will automatically create one visual child for each item in the dataset. Take a look at this simple example: Jeff Tapper Mike Labriola Matt Boles Simeon Bateman
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Here, you have a simple Flex application with only one child, a DataGroup container. The DataGroup is instructed to use a class called DefaultItemRenderer to render each item. You will examine the DefaultItemRenderer and alternatives to it shortly. Next, a dataset is assigned to the DataGroup’s dataProvider property. In this case, the dataset is an ArrayList. In Lesson 8, “Using Data Binding and Collections,” you learned that ArrayCollections not only provide the benefit of data binding but also have a rich set of additional features for sorting, filtering, and finding data quickly. An ArrayList is like an ArrayCollection in that it proxies an Array to provide data binding. Unlike the ArrayCollection, the ArrayList does not provide the additional functionality of sorting, filtering, or searching for items. This is why the ArrayList can be thought of as a lighter-weight version of the ArrayCollection class, concerned only with providing bindability to an underlying Array. Lastly, the DataGroup has its layout set to be vertical. When this is run, four instances of the DefaultItemRenderer will be created, one for each item in the ArrayList. The renderer will use a Label component to show each item.
Implementing an itemRenderer As you saw in the previous example, you tell the DataGroup how to display the elements from its dataProvider by specifying a class to be used as its itemRenderer. In the last example, the DefaultItemRenderer class was used, which simply uses a label to display each element. You can easily create your own itemRenderer as well. When you create your own itemRenderers, your new class can either implement the IDataRenderer interface, or you can subclass a class that already implements that interface, such as the DataRenderer class. The IDataRenderer interface simply dictates that the implementing classes have get and set functions for the data property, which is data-typed generically as an Object. The way the itemRenderer works is that one instance of the renderer will be created for each element in the dataProvider (this isn’t entirely true, but this myth will be exposed later in this lesson, when you learn about virtualization), and the data property of the itemRenderer is set with the data for that element in the dataProvider.
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In this exercise, you will create an itemRenderer that implements the IDataRenderer interface and displays the element in a TextInput instead of a Label. 1 Import the DataGroup.fxp from the Lesson10/independent directory into Flash Builder. Please refer to Appendix A for complete instructions on importing a project. In the DataGroup.mxml file in the default package of the src directory, you will find the code base shown in the previous section. 2 Right-click the src folder of the DataGroup project, and choose New MXML Component. Leave the package blank. Specify the name as TextInputDataRenderer, and set it to be based on spark.components.TextInput. Click Finish.
This will create an MXML file with the following contents:
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3 Add an attribute to the , setting an implements attribute equal to the value mx.core.IDataRenderer.
If you used code completion, a Script block and an import for IDataRenderer will be added for you. If not, add these items manually now. 4 Add a private variable in the Script block, called data, with a data type of Object. private var data:Object;
5 Select the data element, right-click it, and choose Source > Generate Getter/Setter.
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6 Leave the default choices in the Generate Getter/Setter wizard and click OK.
This wizard will create the public get and set functions for the data property and rename the private data to _data. The resulting code will look like this: private var _data:Object; public function get data():Object { return _data; } public function set data(value:Object):void { _data = value; }
7 Add a [Bindable] metadata tag above the get called dataChanged.
data()
function, and specify an event
[Bindable(event=”dataChanged”)] public function get data():Object { return _data; }
As you learned in Lesson 8, this indicates that any elements bound to this class’s data property will be automatically updated when this class dispatches an event named dataChanged.
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8 In the set data() function, dispatch a new event, named dataChanged, after you set the value to the _data property. public function set data(value:Object):void { _data = value; dispatchEvent( new Event( “dataChanged” ) ); }
Your renderer will now dispatch a dataChanged event each time the data property is set, allowing elements that are bound to it to be updated. 9 In the root tag, bind the text property to the toString() method of the data property. Your renderer is now complete. All that remains is to tell the DataGroup to use it. The complete code for the renderer should look like this:
10 Switch back to DataGroup.mxml. Change the itemRenderer of the DataGroup to use your newly created TextInputDataRenderer instead.
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11 Save and run the application. Notice that this time, the elements are rendered as TextInputs, rather than as Labels.
An alternative to implementing the IDataRenderer class yourself is to use a base class, such as the DataRenderer class, that already implements this class. You will do this in the next exercise as you change ProductItem to be a DataRenderer.
Using a DataGroup in the ShoppingView In this exercise, you will switch the VGroup that has the ProductItem instances to be a DataGroup that uses ProductItem as a DataRenderer. 1 Open the ProductItem.mxml from the FlexGrocer project file that you used earlier in this lesson. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer-PreDataRenderer.fxp project from the Lesson10/intermediate folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 In ProductItem.mxml, change the opening and closing tags from Group to be DataRenderer. Add a width=”100%” attribute to the tag. As mentioned earlier, the DataRenderer class is a subclass of Group that implements the IDataRenderer interface. ...
3 In the Script block, override the data setter. In it, set the class’s product property to the value passed to the function. You will need to cast the value as a Product. public override function set data(value:Object):void{ this.product = value as Product; }
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With this small change, your ProductItem class can now function as a DataRenderer. Each time the data property is set, it is in turn passed to the product property, which is already bound to the controls. Next you will change the ShoppingView class to use a DataGroup with your new renderer. 4 Open ShoppingView.mxml. Find the VGroup that contains the three ProductItem instances. Change the opening and closing VGroup tags to be DataGroup tags instead. Remove the three ProductItem instances that are the children.
Next, you will need to specify the dataProvider and itemRenderer. 5 Add an itemRenderer attribute to the opening DataGroup tag, which specifies components.ProductItem as the itemRenderer.
6 Add a dataProvider attribute to the DataGroup, which is bound to the groceryInventory property.
If you save the files and run the application, you will see the products are all rendered on top of each other, with the text being unreadable. This is happening because we haven’t specified a layout object for the DataGroup to use.
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7 As a child tag to the DataGroup, specify a VerticalLayout instance as the value of the layout property.
Now as you save and run the application, the products render properly.
Understanding Virtualization Each visual object takes processor time to create and RAM to store. It is inherently inefficient to create and store visual objects that are not displayed to the user. Virtualization solves this problem by creating visual objects only for the elements that will be seen. If the user needs to scroll to see more elements, they are not created initially; instead, as the user scrolls, the objects that are scrolled off the screen are recycled and reset to display the new elements that are being scrolled on-screen. With virtualization, if a dataset of 1000 items is set in a DataGroup that has room to show 10 renderers, the application will need to create only 10 instances of the renderers rather than 1000, greatly reducing the impact on the processor and RAM.
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To enable virtualization for a DataGroup, you set the useVirtualLayout property of the Layout class to true (it is false by default).
As you know, the layout objects are used by many Flex components, not just DataGroups. However, not all of these support virtualization. If you try to specify a layout to use virtualization in a component that does not support virtualization, the component will simply ignore that attribute of the layout object. In other words, even if you tell the layout of a Group to use a virtual layout, it will still create all its children, visible or not, because Groups don’t support virtualization.
Implementing Virtualization In this exercise, you will take an existing application that has 25 items in a dataProvider of a DataGroup but has room to show only four items at a time, and instruct it to use virtualization. 1 Import the Virtualization.fxp from the Lesson10/independent directory. In the VirtualizedVGroup.mxml file in the default package of the src directory, you will find an application that contains a DataGroup with 25 items in its dataProvider and that uses a variation on the TextInputRenderer you created earlier in this lesson. 2 Run the Virtualization application in Debug mode. Notice in the Console that there are 25 trace statements, one from the creationComplete event of each of the itemRenderers.
As you scroll through the items, you will find you can never see more than five items at any one time (and most times see only four items at a time). However, as you can clearly see in the Console, there are far more than five instances of the TextInputDataRenderer created.
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3 Find the instantiation of the VerticalLayout, and add the attribute useVirtualLayout=”true”. Save and debug the application again. Notice this time, there are only five trace statements of the TextInputDataRenderer instantiated.
Now you can see the real power of virtualization. Rather than having to create an instance of the renderer for each item in the dataProvider, which would be 25 total renderers, only five are created, as that is the most that can be seen in the control at any one time. There is no need to create and keep an additional 20 items in memory; instead, the same five renderers will be used to render whichever items need to be seen at any given time.
Virtualization with Lists With the List class, virtualization is enabled automatically, so you do not need to explicitly tell the layout class to use useVirtualLayout. That much is assumed. In addition to virtualization, Lists also add selectability. Selectability is the idea that the user will be presented with several items and be allowed to choose one or more of them. Lists provide a series of properties, methods, and events surrounding the ideas of selectability. For instance, the selectedIndex and selectedItem properties allow you to specify or retrieve what is currently selected in the list. In this exercise, you will build a renderer to display the various categories shown in the top navigation of the application and specify the list displaying the categories to use that new renderer. 1 Open the FlexGrocer project. 2 Right-click the components folder, and create a new MXML component named NavigationItem. Specify the layout to be VerticalLayout, and the base class to be spark.components.supportClasses.ItemRenderer. Remove the height and width values. ItemRenderer is a subclass of DataRenderer, which additionally implements the methods specified by the itemRenderer interface. These include properties and methods related to displaying which items are and are not selected in a list.
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3 Add an Image tag, specify a height of 31 and a width of 93. Set the source of the image to be assets/nav_{data.name.toLowerCase()}.jpg.
If you look in the assets directory, you will find six files, with names such as nav_dairy.jpg, nav_deli.jpg, and so on. You may notice that the six names are very similar to the names of the categories from the category.xml file, with the difference that the names of the categories in the XML start with an uppercase letter, and in the filenames the categories start with a lowercase letter. To compensate for the difference of the upper- to lowercase letters, invoking the String class’s toLowerCase() method forces the name to be all lowercase, so it can match the case of the file names. After the toLowerCase() method, the category that has a name of Dairy is lowercased and is concatenated into nav_dairy.jpg. 4 After the Image, add a Label whose text is bound to the name property of the data object.
In addition to the image, the desire is to show the category name below the image. 5 Find the VerticalLayout instantiation, and add a horizontalAlign=”center” attribute.
Specifying a horizontalAlign of center will align the image and label horizontally with each other. You now have a functioning renderer that you can use in a List class to display the various categories. 6 Switch back to FlexGrocer.mxml. The List displaying the categories is instantiated in the main application, FlexGrocer.mxml. 7 Remove the labelField attribute from the instantiation of the List in the controlBarContent. Replace that attribute with the itemRenderer for this List to be your newly created NavigationItem class. Change the height property of the List to 52 to compensate for the larger size of the image and text.
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8 Save and run the application. It should now render the images and labels appropriately.
Displaying Grocery Products Based on Category Selection You just passed a dataset to a List control and had an item display for each object in the dataset. At some point you will also want to filter the collection of products to show only the products matching the selected category.
Displaying Grocery Items Based on Category The first step will be to create a filter function in the ProductService class, which will accept a category id and filter the collection to show only the matching products. 1 Open the ProductService class you created in Lesson 9, “Breaking the Application into Components.” 2 Create a private variable named selectedCategory, with a data type of Number, and a default value of 1. private var selectedCategory:Number=1;
3 Create a public function named filterForCategory() that accepts a Product as an argument and returns a Boolean. In the body of the function, return a Boolean indicating whether the catID of the argument matches the selectedCategory property. public function filterForCategory( item:Product ):Boolean{ return item.catID == selectedCategory; }
4 In the handleProductResult() method, after the products ArrayCollection is instantiated, specify a filterFunction() of the products property to use your new filerForCategory() method. Next refresh the products collection. products.filterFunction = filterForCategory; products.refresh();
Now, when the collection is created, the filterForCategory() method is specified as its filter function, and the collection is refreshed, so the filter function will rerun.
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5 Lastly, create a public function named filterCollection() that accepts a numeric argument, named id. Inside the function set the id as the value of the selectedCategory property, and then refresh the collection. public function filterCollection( id:Number ):void{ selectedCategory = id; products.refresh(); }
You now have everything you need in place to filter the collection to a specific category. All that remains is to call the filterCollection() method whenever the category changes.
Adding a Change Handler to the Category List When the user selects an item from a list, a change event is broadcast, indicating that the selected item in the list is no longer the same. In this exercise, you will handle the change event, and pass the id of the selected category to the ProductService to filter the collection so that only matching products are shown. 1 Open FlexGrocer.mxml. 2 Find the List class in the controlBarContent. Add a change handler to the List. Allow code completion to generate a change handler for you.
This will create a method named list1_changeHandler() for you, which accepts an argument named event, of type IndexChangeEvent. This method will automatically be set as the change handler for your list. protected function list1_changeHandler(event:IndexChangeEvent):void { // TODO Auto-generated method stub }
3 Replace the // TODO auto-generated method stub of the list1_changeHandler() with a call to the filterCollection() method of the productService, passing in the id of the selected item from the list (event.target.selectedItem.categoryID). protected function list1_changeHandler(event:IndexChangeEvent):void { productService.filterCollection( event.target.selectedItem.categoryID ); }
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.
What You Have Learned In this lesson, you have: • Populated a List control with a dataset (pages 238–241) • Used a DataGroup with a dataset to display information with an itemRenderer (pages 241–242) • Created an itemRenderer (pages 242–249) • Used the Generate Getter/Setter wizard (pages 242–249) • Learned about virtualization (pages 249–253) • Responded to a user’s choice from a list (pages 253–255)
From the Library of Wow! eBook
LESSON 11
What You Will Learn In this lesson, you will: • Understand the benefits of loosely coupled architecture • Dispatch events • Declare events for a component • Identify the need for your own event classes • Create event subclasses • Create and use a UserAcknowledgeEvent class • Create and use a ProductEvent class • Use event bubbling • Use ProductEvent to add and remove a product • Use the CollectionEvent to update the shopping cart total
Approximate Time This lesson takes approximately 2 hours to complete.
From the Library of Wow! eBook
Lesson 11
Creating and Dispatching Events In previous lessons you worked with events from built-in objects, such as the clicking of a Button or the changing of a List. You may remember that different events all descend from the same Event class but can have more specific information, such as the Mouse position in the MouseEvent. As you get deeper into application development, you will often need to dispatch events that contain your own information. In this lesson, you will learn how to create an event object, set the metadata for the object, and dispatch it. This lesson presents an overview of how to dispatch events within your application, and how to create new Event classes by creating a subclass of Event.
The shopping cart allows you to add and remove items.
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Understanding the Benefits of Loose Coupling At the end of Lesson 10, “Using DataGroups and Lists,” you were left without a way to add or remove items from the ShoppingCart. With your newly refactored application, the buttons for adding and removing are now inside the ProductItem class; however, the ShoppingCart for the whole application is defined within the ShoppingView class. This means that you can no longer directly call the addItem() and removeItem() methods of the ShoppingCart instance. Technically, it would be possible to make the shoppingCart property public and still access the ShoppingCart instance from the ProductItem through an expression like this: this.parent.parent.shoppingCart
However, such an expression can be very problematic for maintaining and debugging the application. During development, refactoring components is often desirable and sometimes essential. If you decide that the DataGroup should be inside another component, perhaps one responsible for all product display functions, the expression above may need to change as well. Over the course of application development, one of two things tends to happen when using these types of expressions. Either you devote an increasing amount of time to maintaining the expressions as the application changes, which slows down progress and makes your day-today work increasingly frustrating. Or, worse yet, you stop refactoring your components even when it makes sense to do so. Maintaining the expressions becomes such a frustrating experience that you simply decide you will never change a specific area of code again. During active development this often leads to workarounds and suboptimal code, and it can often ultimately increase development time and the number of bugs. Both of these ends have a common starting point. An expression like the one above caused ProductItem to have a dependency on ShoppingView. This means that anytime ShoppingView changes, you need to also remember to change ProductItem manually. Creating these types of interdependencies among objects in your application is often called tight coupling, or making a tightly coupled application. In tightly coupled applications, objects often directly modify or access each other’s properties, creating maintenance difficulties. While there will always be some dependency between objects in an application, you need to strive to ensure that those dependencies are appropriate. It is often amusing and useful to think of objects using real-world analogs and models. In the real world, most objects are loosely coupled, which makes tightly coupled examples hyperbolic and fun. Consider a satellite navigation system that provides directions while you drive a car. You and the navigation system exist in a loosely coupled way. When the navigation system is on, it provides
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events indicating whether you should turn or proceed straight. You interpret those events and, ideally, make a decision that it is safe to turn or that you want to proceed in a different direction. Ultimately, you decide whether to engage your muscles and turn the steering wheel. In a very tightly coupled version of this same architecture, the navigation system would take control of your body, forcing your muscles to move as needed to direct you to the new location. As every person is a bit different, the navigation system would have the ability to calibrate the amount of force required per muscle for each individual person. Perhaps it would even need to know your diet or exercise schedule to monitor changes in your musculature to ensure consistent results. The point is simply that the tightly coupled architecture often involves objects having too much information and interacting directly with the internals of other objects. Further, once you make that first concession to make something tightly coupled, you often start down a path of making more and more concessions to make your application work. A loosely coupled architecture strives to use the ideas of notification and interfaces to allow objects to manage themselves more successfully. In application development, maintaining only appropriate coupling can lead to better code reuse, easier refactoring, and the ability to debug an application in parts as opposed to en masse.
Dispatching Events To broadcast an event from a component, you need to use the dispatchEvent() method. This method is defined in the flash.events.EventDispatcher class. Some objects in Flex (for example, the UIComponent), descend directly from EventDispatcher and can dispatch events without any further work. The following is the inheritance hierarchy of the UIComponent class: mx.core.UIComponent extends flash.display.Sprite extends flash.display.DisplayObjectContainer extends flash.display.InteractiveObject extends flash.display.DisplayObject extends flash.events.EventDispatcher
The dispatchEvent() method takes a single argument, which is an event object to be dispatched. When an event is dispatched, anything listening for that event is notified, and any event listeners (handlers) are executed. This offers a way to create a loosely coupled architecture. 1 Import the EventLab.fxp from the Lesson11/independent/ folder into Flash Builder. Please refer to Appendix A for complete instructions on importing a project.
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2 Open and run EventLab.mxml.
The application displays a simple warning dialog with an image, text, and Ok and Cancel buttons. You will use this dialog throughout this lesson, adding events and eventually making the Ok and Cancel buttons function properly. The EventLab application contains two custom components: WarningDialog.mxml and OkayCancelGroup.mxml. These custom components are nested inside the Application, as the following diagram demonstrates:
The WarningDialog is directly inside the Application. It contains an image, text about the warning, and a custom component called OkayCancelGroup. The Ok and Cancel buttons are so often reused together in an application that they have been put into a custom component for this lab. 3 Close your browser and open the OkayCancelGroup.mxml class from the components package. 4 Find the MXML tag for the Ok button. Inside that tag, begin typing the word click. After the third letter, Flash Builder will understand that you intend to handle the click event. Press Enter, and Flash Builder will complete the word click and add the equal sign and quotes. Flash Builder will now prompt you to Generate Click Handler. Either click the option with your mouse or press Enter.
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Flash Builder will generate the block along with a new function named ok_clickHandler(), which accepts a MouseEvent. 5 Repeat step 4 for the Cancel button. When complete, the relevant portion of your OkayCancelGroup should read as follows:
Flex is a general-purpose component framework, so it dispatches general-purpose events. For example, buttons in Flex dispatch a click event when they are clicked. That is a wonderful starting point, but the concept of a click doesn’t mean much in the context of your application. Ultimately, when a button is clicked, it is likely to mean something specific. In this component, when the Cancel button is clicked, it means the user wants to cancel the operation. When the Ok button is clicked, it means the user acknowledges the issue and wants to proceed.
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Therefore, for the purposes of writing code that is much more legible, maintainable, and easier to debug, we often handle events like the click event inside a component, and then immediately dispatch a new, more specific event that makes sense within the context of the application. 6 Inside the ok_clickHandler() method, delete the //TODO comment and create a new instance of the Event object, with the type userAccept. var e:Event = new Event("userAccept");
This creates the new instance of the Event object, which will be dispatched to notify the remainder of the application of the user’s choice. 7 Just after creating the event object, dispatch it. this.dispatchEvent(e);
This dispatches the event so that any listening components can respond as needed. 8 Repeat these steps in the cancel_clickHandler() method, dispatching an event with the type userCancel. var e:Event = new Event(“userCancel”); this.dispatchEvent( e );
9 Save your component and open WarningDialog.mxml from the components package. 10 Find the instance of the OkayCancelGroup component. Inside this tag, inform Flex that you wish to handle the userAccept event with a method named handleAccept(), passing the event object to that method.
Note that you will not get the convenient code hinting and Flash Builder’s ability to generate an event handler for you at this time. In fact, if you save this file presently, you will see an error in the Problems view. That is all right for now. You will examine these issues soon and fix them in the next exercise. 11 Add a new method named handleAccept() to the Script block of WarningDialog.mxml. The method will accept a single parameter named event of type Event. private function handleAccept( event:Event ):void { }
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Tip: You may notice that when you are asked to create a function in this book, it is usually private . However, when Flex creates functions for you, they are usually protected . Both private and protected functions can be accessed by code within a given class, but not by other types of objects in the system . However, when dealing with inheritance, protected functions can be used by subclasses of your objects, whereas private functions cannot . As Flash Builder doesn’t know how your functions will be used, it simply defaults to a more permissive setting . 12 Save WarningDialog.mxml. Look at the Problems view and notice the error. The Problems view is now showing an error: Cannot resolve attribute 'userAccept' for component type components.OkayCancelGroup. While you are dispatching an event in your OkayCancelGroup named userAccept, the Flex compiler, as of yet, is unaware of it. For the compiler to know what userAccept means, you need to perform one additional step, adding metadata to the component that specifically declares any events the component will dispatch. This will also enable code-completion and handler generation in Flash Builder.
Declaring Events for a Component Every component must explicitly declare the events it can dispatch. Components that are subclasses of other components can also dispatch any events that their superclasses have declared. In Flex, events are declared with metadata tags. This is done with the [Event] metadata, which is used to declare events publicly so that the MXML compiler can verify that the user did not simply make a typo. In MXML, an event declaration looks like this: [Event(name=”userAccept”,type=”flash.events.Event”)]
The tag declares that the child elements are all metadata. Next, any metadata is declared. Notice that the tags are enclosed within square brackets. Details for these tags are defined within parentheses. In this example, you can see a userAccept event declared. This event will be an instance of the flash.events.Event class. In this exercise, you will fix the error from the previous exercise by declaring a custom event for the OkayCancelGroup component. 1 Open OkayCancelGroup.mxml from your components package. 2 Before the block, add a metadata block to declare the userAccept event. [Event(name=”userAccept”,type=”flash.events.Event”)]
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If the type is omitted, Flash Builder will assume it is an instance of the flash.events.Event class. While in this case it might save you a few keystrokes, it is usually best to declare the type each time you create a new Event declaration for completeness and additional documentation. 3 Directly below the first Event declaration, but inside the same metadata tag, add a second Event declaration for the userCancel event. [Event(name=”userAccept” ,type=”flash.events.Event”)] [Event(name=”userCancel” ,type=”flash.events.Event”)]
Save OkayCancelGroup.mxml. The errors should now be gone, as Flash Builder understands that this component will be dispatching the named events. 4 Return to the WarningDialog.mxml file and find the OkayCancelGroup tag again. 5 You will now handle the userCancel event. Begin typing userCancel. You will see that Flash Builder now also understands that this component dispatches the userAccept and userCancel events and offers code hinting. Choose the userCancel event and then choose Generate UserCancel Handler.
Flash Builder will add a method named okCancelGroup_userCancelHandler() to your component. The method will automatically accept a parameter of type Event because, due to your metadata declaration, Flash Builder knows what type of event to expect. 6 Add a trace statement to the handleAccept() method that traces the words WarningDialog: Accept to the console.
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7 Add a trace statement to the okCancelGroup_userCancelHandler() method that traces the words WarningDialog: Cancel to the console. Your code should read as follows: private function handleAccept( event:Event ):void { trace( “WarningDialog: Accept” ); } protected function okCancelGroup_userCancelHandler(event:Event):void { trace( “WarningDialog: Cancel” ); }
8 Debug your application. When you click the Ok or Cancel buttons, you should see the corresponding text traced out to the Console view.
9 Terminate your debugging session and return to Flash Builder. You now have a simple reusable component, capable of communicating with other areas of the application in a loosely coupled manner.
Identifying the Need for Custom Event Classes In the previous exercise, events notified other parts of the application about a user action. In addition to notifications, you sometimes need to pass data with events. The flash.events.Event class supports only the properties needed for the most basic style of event, but you are always free to subclass events to make more specific types. Passing data with events is a common practice in Flex and can be extremely beneficial. In this example, you will record a timestamp each time a user clicks Ok or Cancel. This timestamp may be used to log data later, but it is very important that you record the exact time when the user clicks the button. To do that, you are going to create your own event class that will contain this time information. Earlier in this lesson and others, you used the dispatchEvent() method to broadcast an event from a component. The dispatchEvent() method accepts an instance of the flash.events.Event class as its only parameter. Therefore, any events you wish to dispatch must be subclasses of this Event.
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You can add any methods or properties to your event, but you are required to override one method each time you create a new Event. This method, named clone(), is responsible for creating a new event based on the properties of your original event. In other words, it creates an identical clone of it. This method is used by the Flex framework in a number of places, including event bubbling, a concept you will understand before the end of this lesson.
Building and Using the UserAcknowledgeEvent You are about to create an event subclass. This event will have an additional timestamp property, allowing your application to note the exact time when the button was clicked. 1 Right-click the src folder of the EventLab project and create a new ActionScript class. Specify events as the Package name and UserAcknowledgeEvent as the Name of the class. Set the superclass to flash.events.Event.
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Filling out the dialog box automatically creates the skeleton of the class seen here. package events { import flash.events.Event; public class UserAcknowledgeEvent extends Event { public function UserAcknowledgeEvent(type:String, ➥bubbles:Boolean=false, cancelable:Boolean=false) { super(type, bubbles, cancelable); } } }
2 Inside the class definition, create a public property named timestamp to hold a Date instance. public var timestamp:Date;
ActionScript doesn’t have a time data type. However, the Date object can store both date and time information. 3 Change the constructor to accept only two parameters: type, which is a String, and timestamp, which is a Date. In the constructor, pass type to the superclass and store the timestamp in the instance variable you created in step 2. public function UserAcknowledgeEvent(type:String, timestamp:Date) { super(type); this.timestamp = timestamp; }
Like all constructors in ActionScript 3.0, this one is public. The two arguments will be used to populate the event. The timestamp property will be used to hold the time when the event occurred. The type property defines the type of action that occurred to trigger this event (for example userAccept or userCancel). Events often accept two other optional parameters, which you just deleted. We will explore one of those optional parameters later in this lesson. 4 Override the clone() method. In this method, you will return a new instance of the UserAcknowledge event with the same values. override public function clone():Event { return new UserAcknowledgeEvent( type, timestamp ); }
When you override a method in ActionScript 3.0, the method must be defined with exactly the same parameters and return type of the superclass and must include the override keyword. Therefore, the clone() method needs to be defined as public, it must take no parameters, and it must return an instance of the Event class. Your new event is a subclass of the Event class and can therefore be returned by this method.
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The complete UserAcknowledge class should look like the following code block: package events { import flash.events.Event; public class UserAcknowledgeEvent extends Event { public var timestamp:Date; public function UserAcknowledgeEvent(type:String, timestamp:Date) { super(type); this.timestamp = timestamp; } override public function clone():Event { return new UserAcknowledgeEvent( type, timestamp ); } } }
5 Open the OkayCancelGroup.mxml file from your components package. 6 Inside the block, find the ok_clickHandler(). Currently the method dispatches an instance of the Event class. Change this method to instantiate a new instance of the UserAcknowledgeEvent class. Pass userAccept as the first parameter and a new instance of the Date object as the second. Then dispatch this new event. protected function ok_clickHandler(event:MouseEvent):void { var e:UserAcknowledgeEvent = ➥new UserAcknowledgeEvent(“userAccept” , new Date() ); this.dispatchEvent( e ); }
Each time you create a new Date object, it defaults to the current Date and Time. If you used the code-completion feature, an import for UserAcknowledgeEvent was added to your class automatically; otherwise, you will need to manually add the import. import events.UserAcknowlegdeEvent;
7 Repeat this process for cancel_clickHandler(). Instantiate a new instance of the UserAcknowledgeEvent class, passing userCancel as the first parameter and a new instance of the Date object as the second. Then dispatch this new event. protected function cancel_clickHandler(event:MouseEvent):void { var e:UserAcknowledgeEvent = ➥new UserAcknowledgeEvent(“userCancel” , new Date()); this.dispatchEvent( e ); }
You are now dispatching a UserAcknowledgeEvent each time a button is clicked, however, the metadata for this class still indicates that you are dispatching generic Flash
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events. Change the metadata to indicate that each of these events will now dispatch an events.UserAcknowledgeEvent instance. [Event(name=”userAccept”,type=”events.UserAcknowledgeEvent”)] [Event(name=”userCancel”,type=”events.UserAcknowledgeEvent”)]
Your OkayCancelGroup is now broadcasting UserAcknowledgeEvent instances in all cases. All of your code will work properly at this point, and you can run it now to verify this fact. While event times are being stored, you will need to update your WarningDialog.mxml file to output this new information. 8 Open the WarningDialog.mxml file from your components package. 9 Find the handleAccept() method and change the type of the event parameter to events.UserAcknowledgeEvent. private function handleAccept( event:UserAcknowledgeEvent ):void { trace( “WarningDialog: Accept” ) }
If you used the code-completion feature, an import for UserAcknowledgeEvent was added to your class automatically; otherwise, you will need to manually add the import. Previously this method accepted an instance of the Event class. Remember that your UserAcknowledgeEvent is a type of Event, so this code will still work properly. However, Flex knows that the timestamp property does not exist in the Event class. So, to use your new timestamp, this method must be updated to the appropriate type. 10 Update the trace statement by adding a space after the word Accept and before the quotation mark. Then concatenate the event.timestamp to the end of the output. private function handleAccept( event:UserAcknowledgeEvent ):void { trace( “WarningDialog: Accept “ + event.timestamp ); }
11 Repeat this process for cancel, changing the event to type UserAcknowledgeEvent, adding a space after the word Cancel and then concatenating the event.timestamp to the end of the output. protected function okCancelGroup_userCancelHandler(event:UserAcknowledgeEvent): ➥void { trace( “WarningDialog: Cancel “ + event.timestamp ); }
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12 Save and debug the application. It should now output the trace statements along with the time of each click. WarningDialog: Accept Wed Jul 28 15:59:26 GMT-0600 2010 WarningDialog: Cancel Wed Jul 28 15:59:26 GMT-0600 2010
13 Terminate the debugging session.
Understanding Event Flow and Event Bubbling There are two broad categories of classes you have worked with so far in Flex: classes that inherit from DisplayObject and classes that do not. All the Flex UI components such as Lists, Buttons, and Labels are DisplayObjects. The value objects you created, as well as classes like HTTPService, which do not have a visual display, are not DisplayObjects. A lot of work is done in Flex to classes that appear on the screen. The Flash Player and Flex framework need to size and position them on the screen, ensure that they are created at the correct time, and eventually draw them on your monitor. Therefore Flash Player maintains a list, called the display list, of every visual component currently available to the user. So far, every time you have created a visual component, it has been added to the display list. In Lesson 14, “Implementing Navigation,” you will learn to have a little more control over when this occurs. As a mental model, the display list can be thought of as a hierarchy, or tree. Each time something is added to the list, it is added relative to its parent. So, a partial display list for the EventLab application looks like this:
This is only a partial list, as in reality things like Buttons are actually further composed of a label and a background, and so on. Only visual elements are added to the display list, so objects without a display, like the HTTPService, will not appear on this list. This is important, because once something is on the display list, Flash Player provides additional functionality when dispatching events.
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If the event target (the object dispatching the event) is not a visual element, Flash Player simply dispatches the event object directly to the designated target. For example, Flash Player dispatches the result event directly to an HTTPService component. However, if the target is a visual element on the display list, Flash Player dispatches the event, and it travels from the outermost container (the Application container in our simplified example), down through the target component, and optionally back up to the Application container. Event flow is a description of how that event object travels through an application. As you have seen by now, Flex applications are structured in a parent-child hierarchy, with the Application container being the top-level parent. Earlier in this lesson, you also saw that flash.events.EventDispatcher is the superclass for all components in Flex. This means that all visual objects in Flex can use events and participate in the event flow; they can all listen for an event with the addEventListener() method and dispatch their own events. During this trip from the Application to the component that was responsible for the event (known as the target of the event) and optionally back to the Application, other components within the event’s path may listen for and act on the event. In fact, components can decide if they want to listen on the way to the object or on the way back to the Application. The event flow is conceptually divided into three parts: • The capture phase comprises all the components on the trip from the base application to the parent of the event’s target. In other words, everything from the application to the target, not including the target itself. • The target phase occurs when the event reaches the target. • The bubbling phase comprises all the components encountered on the return trip from the target back to the root application. The following image describes a branch of an application in which a Button is contained within a Group, which is contained by a Panel, which sits in the root Application. For the context of this example, other elements in the application are moot.
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If a user clicks the Button, Flash Player dispatches an event object into the event flow. The object’s journey starts at the Application, moves down to the Panel, moves to the Group, and finally gets to the Button. The event object then “bubbles” back up to Application, moving again through the Group and Panel on its way up.
In this example, the capture phase includes the Application, Panel, and Group during the initial downward journey. The target phase comprises the time spent at the Button. The bubbling phase comprises the Group, Panel, and Application containers as they are encountered during the return trip. All instances of the Event class have a bubbles property that indicates whether that event object will participate in the bubbling phase of the event flow. If this property is set to true, the event makes a round-trip; otherwise it ends when the target phase is complete. All of this means that an event can occur in a child component and be heard in a parent. Consider this simple example:
In this case, there is a Button control inside a Group, inside a Panel, inside an Application. When the button is clicked, the click event of the Button control is heard from the event handler of the Button, Group, Panel, and Application, and therefore four Alert boxes pop up, all saying the following: TestApp.ApplicationSkin2._ApplicationSkin_Group1.contentGroup.panel.PanelSkin6. ➥_PanelSkin_Group1.contents.contentGroup.group.button clicked
This string represents the entire path the event traveled in its journey to dispatch. The click event of the Button control can be captured at the Button control itself or in any of the parent containers of the Button instance. This happens because click is a bubbling event. The bubbles property of the Event class is Boolean, which indicates whether an event should bubble. By default, bubbles is set to false on newly created events (although it is preset to true for some built-in events, such as click; you can check the API docs for this information about any event). When you create event instances or event subclass instances, you can decide whether you want to enable bubbling for the event. If you leave the bubbling to the default false value, the event can be captured only at the source of the event (the Button control in the preceding example). However, if bubbling is set to true, the event can be captured by a parent of the dispatching component (such as the Group, Panel, and Application). Currently, the OkayCancelGroup class dispatches an event, and that event is being used by the WarningDialog. Next you will make that event bubble so that it can be handled in the Application itself. 1 Open UserAcknowledgeEvent.as from the events package. 2 Locate the constructor and the call to the superclass. public function UserAcknowledgeEvent(type:String, timestamp:Date) { super(type); this.timestamp = timestamp; }
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The flash.events.Event constructor can accept up to three arguments. The first is the type, which you are passing presently. The second is a Boolean flag indicating whether the event should bubble, and the third is a Boolean indicating whether the event is cancelable (whether another object can cancel the event). 3 Pass true to the bubbles argument of the superclass. public function UserAcknowledgeEvent(type:String, timestamp:Date) { super(type, true); this.timestamp = timestamp; }
This event will now make the return trip (bubble) from the OkayCancelGroup all the way back up to the Application. 4 Open WarningDialog.mxml. The event is now going to pass through this class on the way back to the Application. Therefore, if you choose, you can act on the event here as well. 5 Add an tag and the appropriate event metadata to the WarningDialog indicating that this object will also dispatch a userAccept and userCancel event, both of type events.UserAcknowledgeEvent. [Event(name=”userAccept” ,type=”events.UserAcknowledgeEvent”)] [Event(name=”userCancel” ,type=”events.UserAcknowledgeEvent”)] ... ...
Feel free to copy the entire metadata block from the OkayCancelGroup if you wish to save a few keystrokes.
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6 Open EventLab.mxml. You can now listen for either of these events on the WarningDialog as well as the OkayCancelGroup, as they will bubble up the display list. 7 Inside the WarningDialog tag, begin typing userAccept and userCancel. If the metadata in step 5 was added correctly, Flash Builder will offer you code completion and the ability to generate an event handler. Accept this offer, creating an event handler for both events.
8 Inside the warningDialog_userAcceptHandler(), add a trace statement to output the class name, the event, and the timestamp as follows: protected function warningDialog_userAcceptHandler(event:UserAcknowledgeEvent): ➥void { trace( “EventLab: Accept “ + event.timestamp ); }
9 Inside the warningDialog_userCancelHandler(), also add a trace statement to output the class name, the event, and the timestamp as follows: protected function warningDialog_userCancelHandler(event:UserAcknowledgeEvent): ➥void { trace( “EventLab: Cancel “ + event.timestamp ); }
10 Debug the EventLab application and click both the Ok and Cancel buttons. WarningDialog: Accept Sun Feb 7 13:42:37 GMT-0600 2010 EventLab: Accept Sun Feb 7 13:42:37 GMT-0600 2010 WarningDialog: Cancel Sun Feb 7 13:42:37 GMT-0600 2010 EventLab: Cancel Sun Feb 7 13:42:37 GMT-0600 2010
Notice that the trace statement first occurs in the WarningDialog where the event is received, followed by the EventLab. You have created a bubbling event and handled it in the application. You are now ready to fix the product addition and removal in the FlexGrocer application. 11 Terminate the debugging session and close the EventLab project. You will now move back to the FlexGrocer application and apply this procedure there.
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Creating and Using the ProductEvent Class In this next exercise, you will create an event subclass called ProductEvent. ProductEvent will add a single property to the Event class named product, which will hold an instance of your Product value object. You will then refactor the ProductItem based on your new knowledge of events to reduce some application coupling and restore functionality. 1 Open the FlexGrocer project. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson11/start folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Right-click the src folder and create a new ActionScript class. Set the Package of the new class to events. Name the new class ProductEvent, and set flash.events.Event as the superclass. The skeleton for your new class should look like this: package events { import flash.events.Event; public class ProductEvent extends Event { public function ProductEvent(type:String, bubbles:Boolean=false, ➥cancelable:Boolean=false) { super(type, bubbles, cancelable); } } }
3 Create a new public property for your class, named product, with a data type Product. If you use code hinting and choose the Product class from the list, the import statement for valueObjects.Product will be added automatically; if not, you will need to manually import the class. 4 Modify the constructor for your class so that it takes two arguments. The first argument will remain the type, which is a String. The second argument is an instance of the Product class. public function ProductEvent(type:String, product:Product ) { }
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5 Inside the constructor, pass the type to the superclass, along with a true for the value of the bubbles parameter. Set your local product instance variable equal to the product argument of the constructor. public function ProductEvent(type:String, product:Product ) { super(type, true); this.product = product; }
6 Override the clone() method. This method will return a new instance of the ProductEvent class with the same type and product. public override function clone():Event{ return new ProductEvent(type, product); }
7 Save the ProductEvent class. The class should currently look like this: package events { import flash.events.Event; import valueObjects.Product; public class ProductEvent extends Event { public var product:Product; public function ProductEvent(type:String, product:Product ) { super(type, true); this.product = product; } override public function clone():Event { return new ProductEvent( type, product ); } } }
8 Open ProductItem.mxml from your components package. 9 Remove the public variable named shoppingCart. You are no longer going to attempt to add and remove items from the shopping cart directly. Instead you will use events to inform other components of the user’s actions. 10 Find the addToCart() method. Delete all the existing contents of this method.
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11 Inside the addToCart() method, declare a new local variable named event of type ProductEvent, and set it equal to a new instance of the ProductEvent event class. For the type parameter of the ProductEvent constructor, pass the string addProduct. Then pass the product argument of this method as the second constructor parameter. Finally, dispatch the event. private function addToCart( product:Product ):void { var event:ProductEvent = new ProductEvent( “addProduct” , product ); dispatchEvent( event ); }
If you use code-completion, events.ProductEvent will be imported for you. If not, be sure to import it manually. 12 Repeat this process for the removeFromCart() method, passing the string removeProduct to the ProductEvent type parameter. private function removeFromCart( product:Product ):void { var event:ProductEvent = new ProductEvent( “removeProduct” , product ); dispatchEvent( event ); }
13 Add an tag to this class. Inside it, declare that ProductItem.mxml will dispatch two events named addProduct and removeProduct. Indicate that both events will be of type events.ProductEvent. [Event(name=”addProduct” ,type=”events.ProductEvent”)] [Event(name=”removeProduct” ,type=”events.ProductEvent”)]
14 Save this class and ensure there are no problems in the Problems view. You are now dispatching a bubbling event from the ProductItem when the AddToCart or Remove From Cart buttons are clicked.
Creating a ProductList Component As you learned in previous lessons, you create a custom component in Flex whenever you need to compose new functionality. Previously, you created a DataGroup that displayed products on the screen. While you still want to use that DataGroup, you now need a DataGroup that will dispatch addProduct and removeProduct events. Anytime you make a component’s job more specific, you are talking about subclassing. In this exercise you will subclass DataGroup to make a ProductList. ProductList is a DataGroup with the extra event metadata needed by Flex.
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1 Right-click the components package and choose New > MXML Component. Ensure the package is set to the word components and set the Name to ProductList. Set the layout to spark.layouts.VerticalLayout and the “Based on” field to spark.components.DataGroup, and clear the Width and Height fields. Click Finish.
2 Set the itemRenderer property on the DataGroup node to components.ProductItem.
The DataGroup in your ShoppingView uses components.ProductItem as an itemRenderer. This new ProductList is intended to replace that DataGroup with equivalent functionality plus events.
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3 Add an tag to this class. Inside it, declare that ProductItem.mxml will dispatch two events, named addProduct and removeProduct. Indicate that both events will be of type events.ProductEvent. [Event(name=”addProduct” ,type=”events.ProductEvent”)] [Event(name=”removeProduct” ,type=”events.ProductEvent”)]
This DataGroup is going to use the components.ProductItem renderer. As you declared earlier, that itemRenderer will dispatch two bubbling events: addProduct and removeProduct. As you saw in the EventLab, when an event bubbles, you can listen for the event on any of the parent instances. In this case, you will listen for the addProduct and removeProduct events on the ProductList. 4 Save the ProductList class. It should read as follows: [Event(name=”addProduct” ,type=”events.ProductEvent”)] [Event(name=”removeProduct” ,type=”events.ProductEvent”)]
Using the ProductList Component You will now replace the DataGroup in your ShoppingView with your new ProductList component. 1 Open the ShoppingView.mxml file and locate the DataGroup on approximately line 40. 2 Directly below the DataGroup, add a ProductList component. If you used code-completion, Flash Builder automatically added a components name space on your behalf. If you did not, you will need to add this namespace manually.
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3 Many of the properties on the DataGroup will be the same on your new ProductList. Copy the width, height, and visible properties (for both the normal and cartView state) to your ProductList tag.
4 Finally, move the dataProvider property to the new ProductList and delete the DataGroup. Your new ProductList tag should look like the following code:
5 Save this file and run the application. You shouldn’t receive any errors, and the Products should display as before.
Using ProductEvent to Add and Remove a Product An instance of the ProductEvent class is bubbling up the display list each time the AddToCart button is clicked. You are now going to listen to that event and use it to actually add the product to the cart. 1 Open ShoppingView.mxml from the views package. 2 Inside the Script block, add a new private method named addProductHandler(). This function will accept a single parameter named event of type ProductEvent and return void. Tip: In this case we are writing the event handlers manually . When Flash Builder automatically creates an event handler on your behalf, it normally names it to correspond to the control that is using the event (so, something like productlist1_addProductHandler() if the ProductList were using it) . That is fine in most cases, but this particular handler is going to be used by multiple controls, so we are naming it manually .
3 Still inside the Script block, add another new private method named removeProductHandler(). This function will also accept a single parameter named event of type ProductEvent and return void. private function addProductHandler(event:ProductEvent):void { } private function removeProductHandler(event:ProductEvent):void { }
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If you did not use code-completion, add the import for events.ProductEvent at this time. Again, we are making these methods private as they are not needed outside this class. 4 Inside the addProductHandler() method, create a new local variable named sci of type ShoppingCartItem. Set this variable equal to a new instance of the ShoppingCartItem class, passing the product property of your event object to its constructor. var sci:ShoppingCartItem = new ShoppingCartItem( event.product );
You already did the hard work by ensuring the event would have a reference to the clicked product available. Now you simply need to use it. 5 Still inside the addProductHandler() method, add the ShoppingCartItem instance to the shopping cart using the addItem() method of the shoppingCart reference. Your code should look like this: private function addProductHandler(event:ProductEvent):void { var sci:ShoppingCartItem = new ShoppingCartItem( event.product ); shoppingCart.addItem( sci ); }
6 Duplicate this concept inside the removeProductHandler() method. Create a new local variable named sci of type ShoppingCartItem and assign it a new ShoppingCartItem instance with event.product passed to its constructor. However, in this case, call the removeItem() method of the shoppingCart, passing the local sci variable. private function removeProductHandler(event:ProductEvent):void { var sci:ShoppingCartItem = new ShoppingCartItem( event.product ); shoppingCart.removeItem( sci ); }
You now have two event handlers ready to add or remove products from the cart. You will now simply indicate that these two handlers should be used by your ProductList for this purpose. 7 Find the ProductList tag and indicate that you will handle the ProductList’s addProduct event with the addProductHandler() method, passing the event object.
8 Next, indicate that you will handle the ProductList’s removeProduct event with the removeProductHandler() method, passing the event object.
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9 Save this class and run the FlexGrocer application. You should now be able to add and remove products again using the buttons, but it is now performed with events across components in a loosely coupled way.
Handling the Collection Change Event As you already know, many Flex components and classes, some visual and some non-visual, dispatch events that can be used in your application. In this exercise, you will perform a minor refactoring of the ShoppingCart class and use one of these events to ensure that the total of your ShoppingCart class always remains correct as you add and remove items. 1 Open ShoppingCartView.mxml from the views package. 2 Find the Label tag that displays the text Your Cart Total: $. You will now change this Label to reflect the cart’s actual total. 3 Change the Label to append the total property of the ShoppingCart instance, named shoppingCart, directly after the currency symbol. Surround the expression that retrieves the total in curly brackets, indicating that it should be refreshed if the total changes. Your code should look like this:
Flex will concatenate the initial portion of that string and the total property each time a change in the total is noted. However, there is still one bug in our ShoppingCart class that needs to be fixed. In Lesson 8, “Using DataBinding and Collections,” we added an implicit getter and setter to the ShoppingCartItem. Each time the ShoppingCartItem’s quantity changes, we update the subtotal for that particular item. Unfortunately, the ShoppingCart itself also has a total property. Right now, even though the subtotal for each item adjusts correctly, the ShoppingCart’s overall total is not aware of that change and will therefore not rerun the calculateTotal() method. Effectively, this means that if you update quantities of given items through a method other than add or remove, the ShoppingCart total will not track correctly.
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LESSON 11: Creating and Dispatching Events
4 Open the ShoppingCart class from the cart package. 5 As the last item in the class, add a new method named handleItemsChanged(). This method will accept a single parameter named event of type CollectionEvent. If you used code-completion, CollectionEvent will be imported for you. If not, import mx.events.CollectionEvent now. CollectionEvent is a special type of event broadcast from collections such as the ArrayCollection. It indicates that one of the items in the collection has changed. 6 Inside the named handleItemsChanged() method, call the calculateTotal() method of this object. private function handleItemsChanged( event:CollectionEvent ):void { calculateTotal(); }
Every time the items in the ShoppingCart change, we will respond by recalculating the total for the cart. In this way we can keep track of the changes to the total correctly. 7 Find the constructor for the ShoppingCart class. As the last line of the constructor, you will add an event listener to the items ArrayCollection for the CollectionEvent. COLLECTION_CHANGE event type. When this event occurs you want the handleItemsChanged method called. items.addEventListener(CollectionEvent.COLLECTION_CHANGE, handleItemsChanged );
If you use code-completion, Flash Builder will write much of this line on your behalf. This is simply the ActionScript equivalent of adding an event listener in MXML and passing the event object. The first parameter of the addEventListener() call is always a String specifying the type of event. Unfortunately, in ActionScript, unlike in MXML, Flash Builder doesn’t look at the event metadata and fill in String on our behalf. It is therefore a common convention to create constants in the system, which are just strings with the name of the event preset on your behalf. This simply prevents you from making a typo by ensuring that the event type that you want to listen for does in fact exist. Last thing to note: When you add an event listener in ActionScript, the second argument is a function reference. So you don’t type handleItemsChanged( event ) as you would in MXML, but rather just handleItemsChanged.
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Tip: If you want to see how the constant works for yourself, hold down the Ctrl key and click COLLECTION_CHANGE . Flash Builder will take you to the CollectionEvent class and you will see a constant . This line of code works the same whether you use the constant or type the string collectionChange . 8 Find the addItem() method and remove the call to calculateTotal(). Any change to the items ArrayCollection will now inform the ShoppingCart to recalculate itself. You no longer need to call this explicitly when adding or removing an item. 9 Find the removeItem() method and also remove the call to calculateTotal(). 10 Save this class and run the FlexGrocer application. You can now add and remove items from the cart. As these items change, the total updates automatically as it responds to a notification from the items ArrayCollection.
What You Have Learned In this lesson, you have: • Learned the benefits of loosely coupled architecture (pages 258–259) • Dispatched events (pages 259–263) • Declared events for a component (pages 263–265) • Identified the need for your own event classes (pages 265–266) • Created and used an event subclass (pages 266–270) • Learned about event bubbling (pages 270–275) • Created the ProductEvent class (pages 276–281) • Used ProductEvent to add and remove a product from the cart (pages 281–283) • Used CollectionEvent to update the cart total (pages 283–285)
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LESSON 12
What You Will Learn In this lesson, you will: • Define the viewable columns of a DataGrid through DataGridColumn • Use a labelFunction and an itemRenderer to display DataGridColumn information • Create an MXML component to be used as an item renderer • Create an inline custom item renderer for a DataGridColumn • Raise events from inside an item renderer • Sort the AdvancedDataGrid • Style rows, columns, and cells in an AdvancedDataGrid • Group data in an AdvancedDataGrid using both tags and ActionScript to manipulate the grid’s data provider • Display summary data in an AdvancedDataGrid using both tags and ActionScript
Approximate Time This lesson takes approximately 2 hours to complete.
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Lesson 12
Using DataGrids and Item Renderers In Lesson 10, “Using DataGroups and Lists,” you worked with datasets and some controls that can be used to show the data. In this lesson, you will build on that set of base controls and be introduced to the primary MXML component used to display and manipulate large datasets. In this lesson, you will learn how to use the DataGrid component to display a dataset in an interactive way using rows and columns. Aside from using the DataGrid in its simplest form, you will learn how to override the default behavior of a particular column in the DataGrid by implementing a custom item renderer; do a custom sort of the data in a column; and change the editing controls that manage the underlying data. You will also use the sorting, styling, grouping, and summary data features of the AdvancedDataGrid.
The shopping cart displayed in a DataGrid
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Spark and MX So far in this book we have managed to ignore a fundamental aspect of developing in Flex, which is the use of multiple component sets. Flex 4 is the first version of Flex to have more than one set of components, meaning that there are really two types of Labels, two types of Buttons, and two types of TextInputs, among other controls. These two types are referred to as MX and Spark. The MX set of controls has been under constant evolution since Flex 1.0. The same basic controls, with the same metaphors, have slowly developed over a number of releases. This set of controls worked primarily through the object-oriented concept of inheritance, meaning that if you wanted a control to behave in a new way, you extended the previous control and changed some portion of its functionality. Spark is a brand-new set of controls making their first appearance in Flex 4. In the Spark set of controls, components are built through the concept of composition. You have actually been working with this all along. When you wanted a group to be horizontal or vertical, you combined the Group with a HorizontalLayout or VerticalLayout. This is composition. While Spark is the future of Flex, it is not all-encompassing yet, meaning that the many years of development that went into developing the MX component produced a component set with huge diversity and functionality. Spark has had much less development time, so while it performs extremely well, it does not have breadth of function that MX has today. Therefore in Flex 4, we combine Spark and MX controls to achieve what we desire. Whenever possible, we embrace Spark components, as they are easier to customize, are often more performant, and will be the focus of continuing improvements. When Spark does not have a feature we need yet (such as the Form, DataGrid, and AdvancedDataGrid), we use the MX versions, which integrate nicely with Spark and allow us to complete our component set.
Introducing DataGrids and Item Renderers Using a DataGrid as a way to display the data of your application provides the largest possible number of options for your users to interact with the data. At the simplest level, the DataGrid organizes the data in a column-by-row format and presents this to the user. From there, the DataGrid can be configured to allow you to modify the data it contains. In this lesson, you will make modifications to FlexGrocer, in which the DataGrid will give you a view of the cart and the ability to both update and remove items from the cart.
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Displaying the ShoppingCart with a DataGrid When you left off in Lesson 11, “Creating and Dispatching Events,” you had the contents of your cart displayed in a List control with the ability to remove the current item you were viewing via a Remove From Cart button. You will now use a DataGrid to display the contents of the cart. The DataGrid control supports the syntax that allows you to specify the columns explicitly through the DataGridColumn:
The dataField is used to map a property in the dataset to a given column. The order in which the DataGridColumns are listed is the order you will see the columns from left to right in the DataGrid. This is useful when you need to specify a different order of the columns from the one specified in the dataset. Each DataGridColumn supports a large number of attributes that affect the DataGrid’s rendering and interaction with the given column. 1 Locate the components package that you used in the previous exercise. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson12/start folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Right-click the components package and choose New > MXML Component. In the dialog box, specify the Name to be CartGrid. 3 For the “Based on” value, click the Browse button. In the dialog box, begin to type DataGrid until you see DataGrid – mx.controls displayed. Choose the DataGrid entry. Click OK, then Finish.
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4 In the newly created component’s tag, add the editable property and assign it the value true. You are specifying editable as true because you will allow one of the columns to be changed by the user. If it is set to false, the whole DataGrid becomes read-only.
5 After the tag set, define an tag set. You will be adding DataGridColumn objects in the next steps, and they need to be nested in the columns tag set. 6 In the tag set, define an for the product name. Set the headerText to Product, dataField to product, and editable to false.
The headerText attribute specifies the text of the DataGridColumn header. If you don’t specify this, it will take the value of the dataField attribute. Because the editable attribute is set to true on the tag, you need to set it to false for each column you don’t want to use for editing. 7 Define an for displaying the quantity, and place it after the last . Set headerText to Quantity and dataField to quantity.
This column will be used to allow users to change the quantity of a specific product they want to buy. 8 Define an for displaying subtotals for each item and place it after the last . Set headerText to Amount, dataField to subtotal, and editable to false.
9 Define an for displaying a Remove button. At this point only set editable to false.
Later you will add functionality so a button will remove the item in a particular DataGrid row.
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10 At this point, your component should appear as shown here:
11 Save CartGrid.mxml.
Using the CartGrid Component You’ve created the basic component that uses a DataGrid to display data in the shopping cart data structure. Now you will replace the placeholder DataGrid that was inserted earlier with the newly created component. 1 Open ShoppingView.mxml from the views package. 2 Locate the block near the bottom of the file and remove it. 3 In the same location add the component. Set the id to dgCart, the includeIn to cartView, and the width and height to 100% . 4 In the CartGrid, bind the dataProvider to shoppingCart.items.
5 Run the FlexGrocer.mxml. Add products to the shopping cart, and then click the View Cart button.
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You should see the DataGrid with some data in it. At this point the information is not formatted correctly, but you see that the component is being used and data is being passed to CartGrid. Note the Product column is showing up as text in the DataGrid, even though it is a complex attribute in the dataset. This is because there is a toString() function declared on the Product value object. If this weren’t defined, you would see [Object Product]. You will look at how to better display a complex object later. For now, this is what you should see:
Adding Inline Editing Control for DataGridColumn .
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1 Open the CartGrid.mxml file that you created in a previous exercise. 2 In the tag that maps to the quantity, set the itemEditor to mx.controls.NumericStepper, set editorDataField to value.
This now has the Quantity column being edited as an MX NumericStepper. The NumericStepper control lets the user select a number from an ordered set. The underlying value of the column is bound to the value attribute of the NumericStepper. 3 Save CartGrid.mxml. Run the FlexGrocer application, add the Buffalo product to the shopping cart, and click View Cart. When you click in the Quantity column, you will notice that it doesn’t open as a freeform text field, but rather as a NumericStepper control.
Creating an Item Renderer for Displaying the Product The default behavior of the DataGrid is to convert every value of the dataset to a string and then display it. However, when you are dealing with a complex object that is stored in the dataset, another alternative is to create a custom item renderer that shows more information about the column. In this case, you are going to create a simple item renderer that shows the product’s name and image. When working with item renderers, you will find that there is an implicit public variable available to you in the item renderer called data, which represents the data of the row itself. You can use data to bind your controls without having to worry about what column you are working with. When the DataGrid creates a column that has a custom item renderer associated with it, it creates a single instance of the cell renderer per row, so you don’t have to worry which row of data you are working with in a renderer. You have been using Spark components over MX components whenever possible. Although there is no Spark DataGrid, you will still use Spark components when possible in the item
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renderers. You will use the as the base tag for your item renderers. This class lets you use the Spark item renderer architecture with MX DataGrid. 1 Right-click the components package and choose New > MXML Component. In the New MXML Component dialog box, set the Name to ProductName, the Layout to HorizontalLayout, the base component to an MXItemRenderer; remove any width and height values; and then click Finish. This MXML file will define the layout of a given cell in the DataGrid. You are creating it in a separate file so that, if needed, it can be used on multiple DataGrid columns and/or multiple DataGrids. 2 In the , set the clipAndEnableScrolling attribute to true.
This will ensure that none of the contents of the item renderer extend past the boundaries of the renderer. 3 Place an tag below the tag set to display the product’s thumbnail image. You need to set the source attribute to a hard-coded directory location, but the filename should be bound to the imageName of the product object. That will make it look like {‘assets/’ + (data.product as Product).imageName}. Also assign the width the value of 100.
Tip: The image location used is relative to the location where the main application is loaded from, not the location of the file that contains the tag .
It is worth mentioning here that you could have referred to the image name simply as data.product.imageName. You might have already noticed the first advantage of casting the object as type Product, and that is the code-completion supplied the imageName property when you were typing the code. This would not have happened if you had not done the casting. The next advantage is even more important if you ever make changes to the Product object. The refactoring tools now available in Flash Builder enable you to change the name of the imageName property if you should ever want to. The reference is automatically updated because it is a property of the Product object. Again, this will not happen if you do not do the casting.
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4 Place an tag for the product name below the tag. Bind the text attribute to ( data.product as Product ).prodName. Set the height and width to 100% .
5 Save the ProductName.mxml file. You cannot test this component at this time because it is not yet assigned to the DataGrid. 6 Open the CartGrid.mxml you created in the previous exercise. 7 Update the with a dataField of product with a new attribute, itemRenderer, set to components.ProductName.
With the use of the itemRenderer attribute, you are overriding the default TextInput editor. You need to use the fully qualified class name to set your item renderer unless you have imported the class package that it exists in. 8 Update the with a new attribute variableRowHeight set to true.
It is necessary for you to set the variableRowHeight to true so that Flex resizes the row’s height to accommodate the thumbnail image. Tip: This attribute can be used to allow for exploding details inside a DataGrid row . In this case, you can have summary data in a cell that expands to show details if you click an icon or button .
9 Save CartGrid.mxml. Run the FlexGrocer application, add the Buffalo product to the shopping cart, and click View Cart.
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Creating an Inline MXML Item Renderer for Displaying a Remove Button Another option for creating an item renderer is through the tag, which allows you to declare and create the item renderer inline with the DataGridColumns. From a compiler perspective, doing an inline item renderer is the equivalent of building it in an external file (it actually compiles the code of the inline item renderer as a separate file internally). Inside the tag, you will place an tag, which defines the boundaries of the inline item renderer file from the rest of the page. Thus, the inside of the tag will have its own scope for which you will need to do imports, function declarations, and the like. Tip: Although building inline item renderers will be very efficient from a coding perspective, it does not allow you to reuse the item renderers for other DataGrids . Good candidates are item renderers that are specific to one DataGrid only, such as action item controls . Just like the item renderer you created in the previous exercise, this one will have access to the data variable, which will hold the reference to the row. For this example, you will add a Remove button to each row. 1 Open the CartGrid.mxml you created in the previous exercise. 2 Locate the fourth and change it to a DataGridColumn tag set by removing the “/>” at the end of the tag and adding just the “>” back on.
This is the placeholder column in the DataGrid. We’ll use a start and end tag because the item renderer definition will be placed inside it. You also do not need to specify dataField, because there is no data you are mapping directly to. 3 Place an tag set and an tag set inside the tag.
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4 Place an tag inside the tags to provide a container for the Remove button. In the MXItemRenderer specify a vertical layout with its horizontalAlign property set to center.
When creating this inline item renderer, you want to use the vertical layout to help center the button in the DataGrid no matter the size of the cell. 5 Place an tag after the layout but before the end of the MXItemRenderer. Set the label to Remove and set the click event to call a removeItem() function that you will create momentarily. Pass data as ShoppingCartItem as the parameter to the method. An import statement for valueObjects.ShoppingCartItem should be added automatically to the inline component. If not, add an block inside the tag, and include the import statement.
You need to add the appropriate import statement, because the import statements made at the top of the file are in a scope different from the inline item renderer.
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Reusing the ProductEvent Class At this point there is no removeItem() method in your renderer. When you create this method you will reuse code created in the previous lesson. In Lesson 11 you created an event subclass to hold a Product value object. Now you will reuse that event subclass to dispatch the Product object you want removed from the shopping cart. 1 Inside the block, which is inside the MXItemRenderer you created in the last section, create a private function named removeItem() that returns void and that accepts one parameter, named item, of type ShoppingCartItem. private function removeItem( item:ShoppingCartItem ):void { }
2 Inside the removeItem() method, declare a new local variable named prodEvent of type ProductEvent, and assign it a new instance of the ProductEvent event class. For the type parameter of the ProductEvent constructor, pass the event name removeProduct. Then pass the item.product value as the second constructor parameter. Finally, dispatch the event. public function removeItem( item:ShoppingCartItem ):void { var prodEvent:ProductEvent = new ProductEvent( “removeProduct” , item.product ); dispatchEvent( prodEvent ); }
If you use code-completion, events.ProductEvent will be imported for you. If not, be sure to import it manually. This event will now be dispatched from the itemRenderer and will bubble up toward ShoppingView. 3 Outside the MXItemRenderer, just after the opening DataGrid tag, add an tag. Inside it, declare that CartGrid.mxml will dispatch an event named removeProduct. Indicate that the event will be of type events.ProductEvent. [Event(name=”removeProduct” ,type=”events.ProductEvent”)]
You are now dispatching the Product object you wish removed from the shopping cart. Of course, to actually have it removed you must handle the dispatched event, which you will now do in the next steps. 4 Save the file. 5 Open ShoppingView.mxml. Locate the instantiation of CartGrid you coded earlier in this lesson.
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6 Place your cursor in the tag and press Ctrl-Spacebar to bring up code completion. Select the removeProduct event that you just created in CartGrid. For the event handler call the previously created removeProductHandler() method and pass the event object as a parameter.
The extended event is handled in the CartGrid component. The removeProductHandler() method you built in the previous lesson does the removal of the product from the cart. 7 Run FlexGrocer and add items to the cart. Click the View Cart button and confirm you can remove items from the cart using the Remove button.
Create a labelFunction to Display the Subtotal You need to create a labelFunction to display the subtotal in the third column of the DataGrid. Recall that in Lesson 10 you created a labelFunction to display the product name in a List component. The method signature for a labelFunction on a DataGrid is labelFunctionName(item:Object, dataField:DataGridColumn). 1 In CartGrid.mxml, create a new block. 2 Inside the block, add a private function named renderPriceLabel with the arguments item typed as a ShoppingCartItem and column with the datatype DataGridColumn. The function itself will return a String. private function renderPriceLabel( item:ShoppingCartItem, ➥column:DataGridColumn ):String{ }
If you use code-completion, cart.ShoppingCartItem will be imported for you. If not, be sure to import it manually. Because the DataGrid has multiple columns that can each have its own labelFunction, as well as share the same labelFunction, the second argument is used to distinguish between which DataGridColumn is using the labelFunction. If you know that your function will be used on just one column, you can ignore the second argument in your code. 3 As the first line of code in the renderPriceLabel() function, create a variable local to the function named subtotal with the datatype Number, and assign it the particular column’s dataField value from the item. var subtotal:Number = item[ column.dataField ];
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If you were not creating this function for use by multiple columns, you could have assigned the variable simply as item.subtotal. This would have assigned the correct value. But, since you want the function to be reusable, you use the column name to retrieve the correct data, hence item[ column.dataField ]. 4 As the last line of code in the function, return the subtotal of the item formatted with a $. For now, you want to put a simple mask on the price to represent the number as a dollar figure. The signature and functionality of the labelFunction is the same on the DataGrid as it is on the List. private function renderPriceLabel( item:ShoppingCartItem, ➥column:DataGridColumn ):String{ var subtotal:Number = item[ column.dataField]; return “$” + String( subtotal ); }
5 Update the with a dataField of subtotal with a new attribute of labelFunction set to renderPriceLabel.
This will have the subtotal column use renderPriceLabel on each of the rows in the DataGrid. 6 Check the code for the component you have built. The final code for the CartGrid.mxml should look like the following: [Event(name=”removeProduct” ,type=”events.ProductEvent”)]
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7 Save CartGrid.mxml. Run the FlexGrocer.mxml application, add the Buffalo product to the shopping cart, and click View Cart. Notice both the formatting on the Amount column and the Remove button in the shopping cart.
Using the AdvancedDataGrid The AdvancedDataGrid expands the capabilities of the normal DataGrid. The AdvancedDataGrid control provides added features and greater control of data display, data aggregation, and data formatting. In this section not all features of the AdvancedDataGrid will be used. The less complex features are clearly explained in the Flex user documentation, so the discussion here will cover some of the more conceptually difficult capabilities. At this point, the shopping application does not have a good-use case for the AdvancedDataGrid, so you’ll practice with some smaller files.
Sorting the AdvancedDataGrid In the AdvancedDataGrid, you can now sort by multiple columns, whereas the DataGrid can sort only one column at a time. This behavior differs according to the Boolean value assigned to the AdvancedDataGrid’s sortExpertMode property. When that property is set to false, clicking the header area of a column makes that the first-priority sort. Clicking in the multiple-column sort area adds more sort criteria. The numbers at the top of the columns indicate the sorting order. If you wish to reset the top-level sort, click the header area of a column and that column becomes the first-priority sort. Header areas
Multiple-column sort areas
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1 Import the ADGStart.fxp project from the Lesson12/independent folder into Flash Builder. Please refer to Appendix A for complete instructions on importing a project. You have now imported the project so you can run the applications for AdvancedDataGrid. 2 Run the SortingADG.mxml application. 3 Click the cat header area to sort by product category, and note that the number 1 appears in the multiple-column sort area. By clicking in the header area, you set sorting by category to be the first-priority sort, and the 1 that appears confirms this. 4 Now click in the multiple-column sort area for the name column to make it the secondary sort. You see that the names are now sorted within categories and the number 2 that appears in the name column confirms this is the second-level sort. 5 Click in the qty header area. This changes the first-priority sort to be by quantity. 6 Click in the multiple-column sort area for the qty header. Note that the direction of the arrow in the area changes. By clicking in the multiple-column sort area, you toggle the sort from ascending to descending. 7 Close the browser, return to Flash Builder, and close the SortingADG.mxml file.
Sorting in Expert Mode When you set the sortExpertMode property to true, sorting behaviors change, as well as component visuals. You will not see the multiple-column sort areas. To perform a multiplecolumn sort in this mode, first click in the column header you want for the first-priority sort. Then, Ctrl-click (or Command-click for the Mac) in the header area to add additional sort criteria. The numbers at the top of the columns indicate the sorting order. If you wish to reset the top-level sort, click (not Ctrl-click) the header area of a column, and that column becomes the first-priority sort.
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No multiple-column sort areas available when sortExpertMode set to true
1 Open the application SortingExpertADG.mxml and note the sortExpertMode property is set to true. Run the application. Note that there are no multiple-column sort areas displayed. 2 Click the cat column header to sort the AdvancedDataGrid by product category. Now, Ctrl-click (Command-click) the name column header. Note that when you Ctrl-clicked the name column header, the names were sorted by name within the category. Also, the number 2 appeared in the column header to indicate the sorting order. 3 Ctrl-click again in the name header area. Ctrl-clicking again in a header that already has a sort applied toggles the sort from ascending to descending. 4 Click in the qty column header. This resets the top-priority sort, in this case to the quantity field. 5 Close the browser, return to Flash Builder, and close the SortingExpertADG.mxml file.
Styling the AdvancedDataGrid There are times when you will want to change the look of the AdvancedDataGrid. For instance, you may wish to draw attention to a particular row, column, or cell. A common example of this is to have negative numbers displayed in red. The AdvancedDataGrid allows you to write and apply styling functions to implement this functionality.
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Styling columns Your first task is to change the default look of the grid by applying a style to an entire column. 1 Open the application StyleColumnADG.mxml and run it. Note that there is no special styling on the grid. 2 At the bottom of the Script block, create a new public function named myStyleFunc() which returns an Object. The function should accept two parameters, the first named data, data-typed as Object, and the second named col, data-typed as AdvancedDataGridColumn. Note that the signature of the style function must accept two parameters, the first being an Object and the second an AdvancedDataGridColumn. The first parameter represents the data for a particular row in the AdvancedDataGrid, and the second contains information about the column that the styleFunction property is associated with. The function must return an Object, which is usually one of two kinds of values. The first is null, which means you do not want any styling applied. The function can also return an Object composed of one or more style properties and associated values. 3 In the body of the function, return an Object associating the color property with the hexadecimal value 0xFF0000, and the fontWeight property with the string value bold. Your style function should appear as shown here. public function myStyleFunc(data:Object,col:AdvancedDataGridColumn):Object { return {color:0xFF0000,fontWeight:”bold”}; }
The object returned will be used as a style sheet and applied to part of the AdvancedDataGrid. 4 In the tag that displays the category information, add the styleFunction property and reference the function you just wrote, myStyleFunc. styleFunction=”myStyleFunc”
This will apply the style function you just wrote to the column.
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5 Run the application and note that the category column has red and bold text in it.
6 Close the StyleColumnADG.mxml file.
Styling rows To change the style of a particular row, you also reference a function using the styleFunction property. To change a row, use the styleFunction property with the AdvancedDataGrid tag itself. Then add logic to the function to return the style object only when you want. 1 Open the application StyleRowADG.mxml. Note that the signature for the style function is included and returns null, so no error is reported. Run the application. Note that there is no special styling on the grid. 2 Add the styleFunction property to the tag and have it reference the style function named myStyleFunc.
Adding the style function to the AdvancedDataGrid tag itself causes the style function to be called for every row. For this reason you will add logic to the function so not all rows have the style applied. 3 In the style function, remove the return statement and add an empty if-else statement. public function myStyleFunc(data:Object,col:AdvancedDataGridColumn):Object { if()
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{ } else { } }
The if-else statement will be used to control which row will be styled. 4 For the condition of the if-else statement, check to see if the cost of the data in the current row is .99. if(data[“cost”]==.99)
Remember that the data parameter contains all the data of a particular row of the grid. So, the if statement is checking to see if the cost field for a particular row is .99. 5 If the condition is true, return the styling object {color:0xFF0000,fontWeight:”bold”}, and if the condition is false, return null. public function myStyleFunc(data:Object,col:AdvancedDataGridColumn):Object { if(data[“cost”]==.99) { return {color:0xFF0000,fontWeight:”bold”}; } else { return null; } }
This is the completed style function. 6 Run the application and note the two rows with a cost of .99 are styled.
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NoTe: The style function could be written more concisely, but it may not be as clear . You can take advantage of the fact that an if statement without braces ({}) will execute only the next line of code . Also, once a function returns a value, it does not continue to execute any other code . Using this information, you could rewrite the function to appear as follows: public function myStyleFunc(data:Object,col:AdvancedDataGridColumn):Object { if(data[“cost”]==.99) return {color:0xFF0000,fontWeight:”bold”}; return null; }
Brevity is not always your goal when writing code, so use the approach that is clearest to you and your team .
7 Close the StyleRowADG.mxml file.
Styling cells If you wish to style cells, you still use a style function. You move the styleFunction property back to one of the AdvancedDataGridColumn tags and add logic to return a style only when certain criteria are met. For instance, you may wish to style only the cells that contain the value .99, not the whole row as was just shown. 1 Open the StyleCellADG.mxml file. This contains the same code as the starting file used when styling a row. 2 Implement the same logic in the style function as you did in the last exercise. public function myStyleFunc(data:Object,col:AdvancedDataGridColumn):Object { if(data[“cost”]==.99) { return {color:0xFF0000,fontWeight:”bold”} } else { return null; } }
3 Add a styleFunction property to the tag that displays the cost and references the myStyleFunc function.
This will apply the function only to the cost column of the grid.
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4 Run the application and note only the cells containing .99 are styled.
5 Close the StyleCellADG.mxml file.
Grouping Data Grouping data is an often-requested feature of the DataGrid and is now implemented in the AdvancedDataGrid. The grouping feature allows you to select a data field and group data by that field in a Tree control–like manner. This feature lets you create what is sometimes called a Tree DataGrid because the first column contains an expandable tree to determine which rows are visible in the grid, as shown here.
You have two approaches to implement this functionality, either through ActionScript or via tags nested in the AdvancedDataGrid tag itself. Both methods follow the same approach, which is manipulating the dataProvider of the AdvancedDataGrid.
Grouping data with tags You will first implement grouping using tags. 1 Open the application GroupWithTagsADG.mxml and run it. Note that the category column has been removed, because this is the column you will be grouping on. Notice that all the data are displayed in nongrouped rows.
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2 Remove the dataProvider property from the tag, and insert an tag set just below the tag.
Grouping is implemented by manipulating the dataProvider of the grid. 3 In the dataProvider tag set, nest a GroupingCollection2 tag set, and specify an id property of myGroup. Bind the source property to the dp ArrayCollection.
The GroupingCollection2 class permits you to group data. Its properties permit you to specify both the data to be grouped, as well as how it should be grouped and where it is displayed. 4 Nest an tag set inside the GroupingCollection2 block. Inside the Grouping block, use the tag and set the name property to be cat, which specifies the field on which to group the data. The complete tag block should appear as shown here.
The tag that actually specifies the field to group on is . The name property specifies the field on which to group. 5 Run the application. Notice that no grouping has taken place. In fact, the grid is now not showing any data.
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6 Add a creationComplete event to the AdvancedDataGrid and specify myGroup.refresh() to be executed.
The refresh() method of the GroupingCollection2 class actually applies the grouping to the data. 7 Run the application again, and you will see the data grouped on the category field.
8 Close the GroupWithTagsADG.mxml file.
Grouping data with ActionScript Rather than add tags to the AdvancedDataGrid as you did in the last task, this exercise will now manipulate the dataProvider using ActionScript. The manipulation will take place in a function called initDG, whose skeleton is provided for you. You will later call the function from a creationComplete event on the AdvancedDataGrid. 1 Open the application GroupWithActionScriptADG.mxml and run it. Note that the category column has been removed, because this is the column you will be grouping on. Notice that all the data is displayed in nongrouped rows. 2 In the Script block, following the import of the ArrayCollection class, import the classes you used in the last exercise via tags: the GroupingField, Grouping, and GroupingCollection2 classes. import import import import
mx.collections.ArrayCollection; mx.collections.GroupingField; mx.collections.Grouping; mx.collections.GroupingCollection2;
You will be working with the same classes you did in the previous exercise when you implemented grouping with tags, so you must import them. You could have also used the classes in code and Flash Builder would have made the appropriate imports automatically.
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3 Inside the initDG() function, whose skeleton was provided for you, create a new GroupingCollection2 object named myGroupColl. var myGroupColl:GroupingCollection2=new GroupingCollection2();
The GroupingCollection2 class, and its supporting classes and properties, permits you to implement grouping. 4 Now assign the dataProvider of the AdvancedDataGrid, whose instance name is myADG, to the source property of the myGroupColl object. myGroupColl.source=myADG.dataProvider;
Remember that grouping is basically implemented by manipulating the dataProvider of the grid, so this statement assigns the dataProvider of the AdvancedDataGrid to a property of the GroupingCollection2 so it can be manipulated. 5 Next, create a new Grouping object named group. var group:Grouping=new Grouping();
The Grouping object will act as a holder for a GroupingField object that indicates the grouping specification. 6 Define the field on which to group by creating a new GroupingField object named gf. Pass as a parameter to the constructor the cat field. var gf:GroupingField=new GroupingField(“cat”);
This GroupingField object specifies that the grouping will be implemented on the product category field. 7 Now, using array notation, assign the fields property of the Grouping object the GroupingField object. group.fields=[gf];
The fields property of a Grouping object contains an array of GroupingField objects that specify the fields used to group the data. In this case, you designated the cat field as the grouping field, so this is what is assigned to the Grouping object’s fields property. 8 To complete the creation of the grouping, assign the Grouping object, in this case group, to the GroupingCollection2 object’s grouping property. myGroupColl.grouping=group;
The creation of the grouping collection is complete, but the application will not run correctly without two more lines of code.
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9 As the last two lines of the function, use the refresh() method to set the grouping on the GroupingCollection2 object, and then assign the GroupingCollection2 object back to the AdvancedDataGrid’s dataProvider. myGroupColl.refresh(); myADG.dataProvider=myGroupColl;
You must refresh the GroupCollection using the refresh() method, then assign the GroupingCollection2 object to the dataProvider of the AdvancedDataGrid before you will see any results in the application. 10 Be sure your initDG() function appears as shown here. private function initDG():void { var myGroupColl:GroupingCollection2=new GroupingCollection2(); myGroupColl.source=myADG.dataProvider; var group:Grouping=new Grouping(); var gf:GroupingField=new GroupingField(“cat”); group.fields=[gf]; myGroupColl.grouping=group; myGroupColl.refresh(); myADG.dataProvider=myGroupColl; }
You’ve built the function to implement grouping, but as of yet you have not called the function. 11 Add a creationComplete event to the tag and call the initDG() function.
The function that manipulates the dataProvider will now be called to implement grouping. 12 Run the application and confirm that grouping on the product categories is working just as it did when implemented in the previous exercise with tags.
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13 Close the GroupWithActionScriptADG.mxml file. To review, here are the steps you just completed to group via ActionScript: 1. Create a GroupCollection object. 2. Assign to the source property of the GroupCollection object the AdvancedDataGrid’s dataProvider. 3. Create a new Grouping object. 4. Create a new GroupingField object that specifies the field on which to group. 5. Assign an array of GroupingField objects to the fields property of the Group object. 6. Assign the Grouping object to the grouping property of the GroupingCollection2. 7. Refresh the GroupingCollection2. 8. Assign the GroupingCollection2 to the dataProvider of the AdvancedDataGrid.
Displaying Summary Data Often when displaying data in a table, you want to display summaries of that data. For instance, when displaying sales data by region, you may want to display summary data for each of the regions. This is now possible with the AdvancedDataGrid. Just as with the grouping of data, you can create summary data with tags or ActionScript. Because you can display summary information for data represented only by the GroupingCollection2 class, you will work with the code you finished in the previous exercises where grouping was correctly implemented. The basic concept of summary data is shown in the following figure:
Here the summary shows the number of bakery items purchased is five.
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Displaying summary data with tags In this exercise, you will use the working grouping example and add summary data. You will implement summary information with tags, and in the next exercise, you will implement the same functionality using ActionScript. 1 Open the application SummaryWithTagsADG.mxml and run it. You see that grouping is functioning on the category field. 2 Add a fourth AdvancedDataGridColumn to display a field called summary, which you will create shortly.
This will add a column to contain the summary data. You will create the summary field using tags. 3 Change the GroupingField tag into a tag set with opening and closing tags.
The summary tags must be nested inside the GroupingField tag block. 4 Insert an tag set nested in the GroupingField block. You create summary data about your groups by using the summaries property of the GroupingField class. 5 Insert an tag set nested in the summaries block. Add to the opening SummaryRow tag a summaryPlacement property, set equal to last.
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The summaryPlacement property specifies where the summary row appears in the AdvancedDataGrid control. Your options are: •
first:
•
last:
•
group:
Create a summary row as the first row in the group.
Create a summary row as the last row in the group.
Add the summary data to the row corresponding to the group.
.
So, by specifying last as the option, the summary information will be displayed after all the products in a specific category.
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6 In the SummaryRow block, nest an tag pair. In that tag set, nest an tag. In the SummaryField2 tag, set the following three properties to the indicated values: •
dataField:
qty
•
summaryOperation:
•
label:
SUM
summary
The fields property of the SummaryRow class holds one or more SummaryField2 objects. The SummaryField2 objects define how the summary should be created. The dataField property defines on which data field the summary will be computed. The summaryOperation defines how the summary data should be computed. Valid values are: • SUM • MIN • MAX • AVG • COUNT Tip: If those operations do not perform the calculation you need, you can use a summaryFunction property to specify a function to compute a custom data summary .
The label property associates the summary value to a property. In this case, the property name is summary, which corresponds to the fourth AdvancedDataGridColumn you added at the start of this exercise.
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7 Check to be sure your AdvancedDataGrid appears as shown here:
8 Run the application. You will see that summary data is displaying as the last line of the category group. The sum of the quantities in that group is the summary information displayed.
9 Do not close the file at this time.
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Changing the display using rendererProviders You have displayed the summary information, but most likely not in the way in which you would choose. To display the data in a more readable format, you must use rendererProviders. Previously in this lesson, you used item renderers for the normal DataGrid and assigned them to individual columns. In the AdvancedDataGrid, you assign the renderers to the AdvancedDataGrid itself, and then specify where to use the renderers. The first step is to build a simple renderer. 1 Right-click the renderers package and then choose New > MXML Item Renderer. Enter renderers as the Package and SummaryText as the Name, and use the Item renderer for XM AdvancedDataGrid for the template. Click Finish. This creates the basis for the renderer you will create to display summary information. 2 Alter the text property in the Label tag to be Total number of items: {data.summary} for the property’s value. The entire component should appear as follows: >
You access the summary information through the data property. Remember that the data property contains all the data for a particular row in the grid and is automatically passed to the component by Flex. 3 Return to the SummaryWithTagsADG.mxml file and just above the closing AdvancedDataGrid tag, insert an tag set. In the rendererProviders tag set, insert an tag. In the AdvancedDataGridRendererProvider, set the following four properties to the values indicated. •
dataField:
summary
•
columnIndex:
•
columnSpan:
•
renderer:
1
2
renderers.SummaryText
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Your code should appear as shown here:
In this case, the rendererProviders property contains only one definition of an AdvancedDataGridRendererProvider, but the property can contain one or more definitions. This particular renderer is tied to the summary field, and the columnIndex property will cause the renderer to be displayed in the first column, where columns are zeroindexed. The columnSpan property will cause the renderer to span two columns. The renderer property that indicates the component to use is in the renderers directory and is named SummaryText. Tip: When the columnSpan property is set to 0, the renderer spans all columns in the row . NoTe: You could have placed this block of code anywhere in the AdvancedDataGrid tag block as long as it wasn’t nested in a tag set other than AdvancedDataGrid . For instance, the block of code could have also been placed just below the opening tag or just above the tag block .
4 Remove the column that displays the summary. You no longer need this column because you specified where the renderer should be displayed. Leaving in this column would cause the summary data to be displayed by both the renderer and the column. 5 Run the application to see the renderer in action.
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Tip: If you do not like the document icon in front of each product, you can bind the defaultLeafIcon to null to remove it .
6 Close the SummaryWithTagsADG.mxml file.
Displaying summary data with ActionScript Just as you implemented grouping with both tags and ActionScript, so you can implement summaries with both tags and ActionScript. Use as a starting file the code that implemented grouping with ActionScript, plus the AdvancedDataGridRendererProvider you created in the previous exercise. 1 Open the SummaryWithActionScriptADG.mxml application. 2 In the Script block following the import of the ArrayCollection class and the grouping classes, import the SummaryField2 and SummaryRow classes you used in the last section via tags. import import import import import import
mx.collections.ArrayCollection; mx.collections.GroupingField; mx.collections.Grouping; mx.collections.GroupingCollection2; mx.collections.SummaryField2; mx.collections.SummaryRow;
You could have also just used these classes in code, and Flash Builder would have imported them for you. 3 Inside the initDG() function, locate where the GroupingCollection2 object, named myGroupColl, invokes the refresh() method. Create a few blank lines just above this code. This is where you must enter the code to create the summary information. In this location, create a SummaryRow instance named sr and a SummaryField2 instance named sf. var sr:SummaryRow=new SummaryRow(); var sf:SummaryField2=new SummaryField2();
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Both of these objects are needed, just as you needed them when implementing summaries with tags. 4 Now, assign the sf object the following three properties and associated values: •
dataField:
qty
•
summaryOperation:
•
label:
SUM
summary
sf.dataField=”qty”; sf.summaryOperation=”SUM”; sf.label=”summary”;
These values are assigned for the same reasons mentioned when implementing summaries with tags. 5 Using array notation, assign the SummaryField2 object to the fields property of the SummaryRow object. sr.fields=[sf];
The fields property of a SummaryRow object contains an array of SummaryField2 objects. These objects specify the fields used to compute summary information. 6 Set the summaryPlacement property of the SummaryRow object to the value last. sr.summaryPlacement=”last”;
The values for the summaryPlacement property are the same as discussed when implementing summaries with tags. 7 As the last line of code needed to create a summary, assign the summaries property of the GroupingField object, in this case named gf, the SummaryRow object, using array notation. gf.summaries=[sr];
The summaries property contains an array of SummaryRow instances that define the summaries to be created. 8 Check to be sure your initDG() method appears as shown here, paying special attention to the code just added to implement summary information.
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private function initDG():void { var myGroupColl:GroupingCollection2=new GroupingCollection2(); myGroupColl.source=myADG.dataProvider; var group:Grouping=new Grouping(); var gf:GroupingField=new GroupingField(“cat”); group.fields=[gf]; myGroupColl.grouping=group; var sr:SummaryRow=new SummaryRow(); var sf:SummaryField2=new SummaryField2(); sf.dataField=”qty”; sf.summaryOperation=”SUM”; sf.label=”summary”; sr.fields=[sf]; sr.summaryPlacement=”last”; gf.summaries=[sr]; myGroupColl.refresh(); myADG.dataProvider=myGroupColl; }
9 Run the application, and you will see the summary information just as it appeared when using tags.
10 Close the SummaryWithActionScriptADG.mxml file.
What You Have Learned In this lesson, you have: • Displayed a dataset via a DataGrid (pages 289–292) • Defined the viewable columns of a DataGrid through DataGridColumn (pages 292–293) • Created an MXML component to be used as an item renderer (pages 293–295) • Created an inline custom item renderer for a DataGridColumn (pages 296–299)
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• Displayed information from a DataGridColumn using a labelFunction and an item renderer (pages 299–302) • Learned how to raise events from inside an item (page 302) • Sorted the AdvancedDataGrid in two different ways (pages 302–304) • Applied custom styling to rows, columns, and cells of the AdvancedDataGrid (pages 304–309) • Manipulated the dataProvider for an AdvancedDataGrid to group data in an AdvancedDataGrid (pages 309–314) • Created summary information for data in an AdvancedDataGrid (pages 314–323)
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From the Library of Wow! eBook
LESSON 13
What You Will Learn In this lesson, you will: • Learn the terminology associated with drag-and-drop operations in Flex • Understand that the list-based components in Flex have enhanced drag-anddrop support built in • Implement drag and drop on drag-enabled components • Use various drag events • Implement various methods of the DragSource and DragManager classes to implement drag and drop on non-drag-enabled components • Use formats to allow the dropping of drag proxy objects
Approximate Time This lesson takes approximately 1 hour and 30 minutes to complete.
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Lesson 13
Using Drag and Drop Drag and drop is a common user interface technique in desktop applications. It was not so, however, in web applications until the rich Internet applications (RIAs) came along. Flex and Flash Player permit you as a web developer to use drag and drop just as a desktop developer does. To implement drag and drop in a Flex application, you use the Drag and Drop Manager and the tools it provides. The Drag and Drop Manager enables you to write a Flex application in which users can select an object and then drag it to, and drop it on, a second object. All Flex components support drag-and-drop operations, and a subset has additional drag-and-drop functionality in which implementation requires little more than adding a single property. In this lesson, you will implement drag and drop in your e-commerce application so a user can click a product and drag it to the shopping cart.
Dragging a grocery item to the shopping cart
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LESSON 13: Using Drag and Drop
Introducing the Drag and Drop Manager The first step in understanding the Drag and Drop Manager is to learn the terminology surrounding it. The terminology is summarized in the following table. Drag and Drop Manager Terminology Term
Definition
Drag initiator
Component or item from which a component is being dragged .
Drag source
Data being dragged .
Format
Property of the DragSource that allows an object to be dropped, or not, on another object . The data in the DragSource is also associated with the format . The data type of the formats are simple strings .
Drag proxy
Image displayed during the dragging process .
Drop target
Component that the drag proxy is over .
The following figure gives you a visual representation of the terminology: Drag source (data being dragged—non-visual)
Drop target (dragging to)
Drag initiator (dragging from)
Drag proxy (image during the drag)
There are three phases to a drag-and-drop operation: 1. Initiating: A user clicks a Flex component or an item in a Flex component and then begins to move the component or item while holding down the mouse. The component or item is the drag initiator.
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2. Dragging: While holding down the mouse button, the user moves the mouse around the screen. Flex displays an image called a drag proxy, and the associated non-visual object called the drag source holds the data associated with the component or item being dragged. 3. Dropping: When the user moves the pointer over another component that will allow it, the item can be dropped on a drop target. The data is then inserted into the new component in some way. Flex components fall into two groups when it comes to drag-and-drop support: those with enhanced drag-and-drop functionality and those without. The following list-based controls have enhanced support for drag and drop: •
•
•
•
•
•
•
•
What this means to you as a developer is that your life will be a little bit easier when implementing drag and drop with those controls that have enhanced support. In fact, in many cases that might require no more than setting a single property value for each of the controls involved in the drag-and-drop operation.
Dragging and Dropping Between Two DataGrids Your first foray into implementing drag-and-drop operations in Flex will be between two DataGrids. Because they are list-based components and have enhanced drag-and-drop support, you will need to write very little code. Two properties are important in this first phase: dragEnabled and dropEnabled. Here are their descriptions: •
dragEnabled:
Assigned a Boolean value to specify whether the control is allowed to act as a drag initiator (defaults to false). When it’s true, the user can drag items from the component.
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•
dropEnabled:
Assigned a Boolean value to specify whether the control is allowed to act as a drop target (defaults to false). When it’s true, the user can drop items onto the control using the default drop behavior.
Stated most simply, you set the dragEnabled property in the component from which you are dragging to true, and set the dropEnabled property in the component on which you are dropping to true. So now you will put your drag-and-drop knowledge to use by implementing drag and drop from one DataGrid to another DataGrid. 1 Import the DragDropStart.fxp from the Lesson13/independent/ folder into Flash Builder. Please refer to Appendix A for complete instructions on importing a project. 2 Open the Task1_DG_to_DG.mxml file. You will use this project instead of the FlexGrocer one because some of the work in this lesson will not be directly involved with the FlexGrocer site. 3 Examine the code in the Task1_DG_to_DG.mxml file, and then run it. Note that the existing code does not use any concepts you have not already learned in this book. The file uses an HTTPService remote procedure call (RPC) to retrieve grocery info. The file then uses a result handler to place the data into an ArrayCollection, which is then used as a dataProvider in a DataGrid. When you run the application, you see you have a DataGrid populated with grocery product information and another DataGrid below it. Try to drag and drop between the DataGrids; you will see that this functionality is not yet working. 4 In the first DataGrid, set the dragEnabled property to true. Run the application; you can click one of the rows in the DataGrid and drag the drag proxy around the screen. Setting this property did two obvious things: It enabled dragging and created the drag proxy, the image attached to the pointer when dragging. Another non-visual event occurred at the same time: A DragSource object was created to hold the data. The data is associated with a format named items, as the following figure from the debugger shows:
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DragSource object
Actual data here
5 In the block below the existing variable declaration, create a bindable private variable named targetGridDP of data type ArrayCollection and assign it a new ArrayCollection. Then bind this variable as the dataProvider of the second DataGrid, whose id is targetGrid. [Bindable] private var targetGridDP:ArrayCollection=new ArrayCollection(); … >) in the upperright corner of Flash Builder and selecting that perspective. Normally, the Flash perspective is best to work in because you can see so much more of your code. 8 As the third line of code in the function, add the name of the product to the List by using the addItem() method of the List’s dataProvider. Remember that the dgRow variable contained an array of length 1, so use dgRow[0].name to reference the name. targetList.dataProvider.addItem(dgRow[0].name);
This is a case in which viewing how the data is stored using the debugger is very helpful in retrieving the information. 9 Run the application and drag from the DataGrid to the List. You should see the product being placed in the List, but [object Object] also appears. The event continued to do what it was supposed to do, even though you displayed some different data; hence, you still see the reference to the object.
10 As the last line in the function, use the event class’s preventDefault() method to cancel the event default behavior. event.preventDefault();
In this case, you can cancel the default behavior. Not all events can be canceled; you must check the documentation for definitive answers on an event-by-event basis. By canceling this event, you prevent the display of [object Object] in the List. 11 Run the application. When you drag from the DataGrid to List, only the name of the product appears in the List. This wraps up our second task in this lesson on drag and drop.
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Using a Non-Drag-Enabled Component in a Drag-and-Drop Operation
Description addData(data:*.
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Description
.
Tip: The doDrag() method has a number of optional parameters to control the look of the drag proxy . You can find these parameters in the Class documentation for DragManager in the Adobe Flex 4 Language Reference .
Now you’re ready to start writing code for this exercise. 1 Examine the code in the Task3_Label_to_List.mxml file, and then run it. You see you have a Label with the text “Drag me” in it and an empty List below it. At this point, there is no drag-and-drop functionality.
2 At the top of the Script block import the four classes shown here that you need in the application: import import import import
mx.core.DragSource; mx.managers.DragManager; mx.events.DragEvent; mx.core.IUIComponent;
You could have also just used these classes as data types, and Flash Builder would have imported them for you automatically.
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3 In the Label, add a mouseDown event and have the event call a function named dragIt(). The function call should pass four parameters; the first is the drag initiator, which in this case is the instance name of the Label: myLabel. The second parameter is the data you will later place in the DragSource object. In this case, just pass a string of My data here. The third parameter is the event, which of course is just event. The last parameter is the format that will be associated with this data. In this task, use myFormat. mouseDown=”dragIt(myLabel, ’My data here’ ,event, ’myFormat’)”
This is the function that will be called to initiate the drag-and-drop operation. You need to pass the parameters because they are all needed in the function to allow: • Dragging to start • Placing the data in the DragSource object associated with the format 4 At the bottom of the block, create a private function named dragIt(), which returns void. The function should accept four parameters that, of course, correspond to the data passed to the function. Use the names and data types shown here: initiator:Label dsData:String event:MouseEvent format:String
Of these parameters, the initiator could be any kind of component, and the dsData could be nearly any kind of data you want to be dragged from one component to another. The event will often be the mouseDown MouseEvent or the mouseMove event, but that would not change either the event parameter name nor the data type used here. The format will always be a String. 5 As the first line of code in the function, create a variable local to the function named ds typed as a DragSource and set it equal to a new DragSource object. var ds:DragSource=new DragSource();
This creates the DragSource object that will have data added to it. 6 Next in the function, use the addData() method of the ds DragSource object to add the data passed in the dsData parameter to the ds object. Associate it with the format passed in the format parameter. ds.addData(dsData,format);
An important point here is that you can store data associated with multiple formats, which means you can use multiple addData() methods on the same DragSource object.
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You might want to do this if you have multiple drop targets and want to drop different data in each drop target. The different drop targets would use different arguments in the dataForFormat() method to get the appropriate data. 7 As the last line of code in the function, permit the Label to be dragged by calling the static doDrag() method of the DragManager class. You pass it the three arguments initiator, ds, and event. Check to make sure your completed function appears as shown here: private function dragIt(initiator:Label,dsData:String,event:MouseEvent, ➥format:String):void{ var ds:DragSource=new DragSource(); ds.addData(dsData,format); DragManager.doDrag(initiator,ds,event); }
Remember that a static method is one you can invoke directly from the class without first instantiating it. 8 Run the application and drag the Label. At this point there is no drop target that will accept the Label. You now move on to coding the List to accept the drop of the Label and to display the data passed in the DragSource in the List. 9 In the List, add a dragEnter event and have it call a function named doDragEnter(). The function should pass two parameters. The first is the event, and the second is the format—which in this case should match the format used earlier: myFormat. dragEnter=”doDragEnter(event, ’myFormat’)”
You are passing data to the function that allows the initiator, the Label, to be dragged to the drop target, the List. 10 At the bottom of the block, create a private function named doDragEnter(), which returns void. The function should accept two parameters. Name the first parameter event, typed as a DragEvent, and the second parameter format, typed as a String. private function doDragEnter(event:DragEvent,format:String):void { }
Both these parameter values are needed to allow the dropping of the initiator.
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11 Insert into the function an if statement that checks to see whether the formats of the two objects match. Use the hasFormat() method of the DragSource object, which is contained in the event object. The argument of the hasFormat() method should be the format parameter passed to the function. Remember the hasFormat() method returns true if the DataSource object contains a matching format of the drop target; otherwise, it returns false. if(event.dragSource.hasFormat(format)){ }
The List is looking in the DragSource object and seeing whether a format exists that matches one of the formats it is allowed to accept. The hasFormat() function will return either true or false. 12 If the hasFormat() function returns true, use the DragManager’s static function of the acceptDragDrop() method to allow the dropping. The argument of the function should be the List itself, which is best referred to in this case as event.target. DragManager.acceptDragDrop(event.target);
You could have actually replaced event.target with the instance name of the List, myList, and the function would have had the same result. The advantage of using the more generic event.target is that it makes this function more reusable. You can use the function for any dragEnter result handler—it will work correctly. 13 The acceptDragDrop() method is defined to accept an object of type IUIComponent. For this reason you need to cast event.target as an IUIComponent to satisfy the compiler. The IUIComponent class defines the basic set of APIs that must be implemented to be a child of a Flex container or list. 14 Be sure that the new function appears as follows, and then run the application. private function doDragEnter(event:DragEvent,format:String):void{ if( event.dragSource.hasFormat( format )){ DragManager.acceptDragDrop( event.target as IUIComponent ); } }
You should now be able to drag the Label. When it moves over the List, the red X disappears, and you can drop the drag proxy. At this point, nothing happens when you do the drop. 15 In the List, add a dragDrop event and have it call a function named doDragDrop(). The function should pass two parameters, the event and the format, which in this case should match the format used earlier: myFormat. dragDrop=”doDragDrop(event, ’myFormat’)”
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You are passing the data needed to have the data retrieved from the DragSource and have it displayed in the List. 16 At the bottom of the block, create a private function named doDragDrop(), which returns void. The function should accept two parameters. Name the first parameter event, data typed as DragEvent, and the second parameter format, typed as String. You need the event object in this function because it contains the DragSource object, and that is where the data is stored. Remember that you stored the String My data here in the DragSource object in steps 3–6 of this exercise. The format is needed because that is how you pull data from the DragSource object using the dataForFormat() method. 17 As the first line of code in the new function, create a variable local to the function named myLabelData, typed as Object. Assign this variable the data being dragged by using the dataForFormat() function to retrieve the data from the dragSource property of the event object. The argument of the function should be the format parameter passed to the function. var myLabelData:Object=event.dragSource.dataForFormat(format);
Remember that you can store data associated with multiple formats, so you must specify which format’s data to retrieve when retrieving data. 18 Display the data just retrieved in the List. You need to use the addItem() method on the List’s dataProvider property to do this. myList.dataProvider.addItem(myLabelData);
You have achieved your goal of moving the Label’s data into the List. 19 Be sure that the new function appears as follows, and then run the application. private function doDragDrop(event:DragEvent,format:String):void{ var myLabelData:Object=new Object(); myLabelData=event.dragSource.dataForFormat(format); myList.dataProvider.addItem(myLabelData); }
Now when you drag the Label to the List, you will see that the data from the Label, the String “My data here” is displayed in the List. The following figure shows the List after successfully dropping the Label data.
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Now that you have a solid background in drag and drop, you will implement drag-and-drop functionality in the e-commerce application of FlexGrocer.
Dragging a Grocery Item to the Shopping Cart The culmination of your work in this lesson is to implement dragging a grocery item into the shopping cart. The exercises you have performed so far in this lesson have prepared you well for this final exercise; in fact, some of the code you have already written will be copied and pasted for use in this exercise. In these steps, you will enable the user to click the grocery item, drag it to the small shopping cart, and then drop it in. The grocery item is displayed in a VGroup container, and the shopping cart is a List. Because the VGroup is not a drag-and-drop-enhanced component, you will have to pattern your code here after what you just wrote in the section “Using a Non-DragEnabled Component in a Drag-and-Drop Operation.” 1 Open the FlexGrocer project you used in the previous lesson. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson13/start folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Open ProductItem.mxml from the components package. This is the component in which the grocery data is displayed; so this is where you will have to permit the data to be dragged. Tip: At first, you will drag all the data to the shopping cart and then write the code so that just the image of the item acts as the drag proxy . 3 Locate the just below the tag set. In that container, locate the tag that displays the product. Add an id property to the Image tag and assign it the value img. You will be referencing this image several times and need to give it an instance name. 4 Add a mouseDown event in the Image tag and select Generate DragDrop handler to have Flash Builder create the event handler for you. Note that the event object was automatically passed. Add a second parameter, which is the format that will be associated with this data. In this task, use the string cartFormat. mouseDown=”img_mouseDownHandler(event, ’cartFormat’)”
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LESSON 13: Using Drag and Drop
By placing the mouseDown event on the Image, it will enable the user to start the drag process by clicking the image. 5 Locate the newly created event handler in the block and remove the TODO comment. Add a second parameter named format typed as a String. protected function img_mouseDownHandler(event:MouseEvent,format:String):void{ }
This function has two main purposes: to get data into the object being dragged and to permit the component to be dragged. 6 As the first line of code in the event handler, create a new variable local to the function named dragSource and assign it a new DragSource object. Next, use the addData() method to associate the newly created object with the product and format. var dragSource:DragSource=new DragSource(); dragSource.addData(product,format);
If you used code-completion, DragSource was imported for you. Else, import mx.core. DragSource manually. Remember that the addData() method’s two parameters assign the data and the format to the DragSource object. In this case the data is the product being displayed in the VGroup, and the format is the format string passed to the event handler. 7 As the last line of code in the function, permit the Image to be dragged by calling the static doDrag() method of the DragManager class. Recall that you must pass the method three parameters, the initiator, the DragSource object, and the event. In this case the initiator is event.target as IUIComponent. The DragSource is the dragSource object you created. Lastly, the event is the event parameter passed to the event handler. DragManager.doDrag(event.currentTarget as IUIComponent, dragSource, event);
If you used code-completion, IUIComponent and DragManager were imported for you. If not, import mx.core.IUIComponent and mx.managers.DragManager now. You had to cast the initiator as IUIComponent because event.currentTarget is typed as an Object by default, which is not a visual component and hence cannot be dragged. 8 Run the FlexGrocer.mxml application. You should be able to drag the grocery item data. You see the drag proxy is the outline of the Image, or a rectangular box. Later in this task, you will change the drag proxy to the image of the grocery item. At this point there is no drop target, so you cannot drop the data anywhere. 9 Open the ShoppingView.mxml from the views package. This file contains the List that is your shopping cart to which grocery items are dragged.
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10 Locate the List with the id of cartList and add a dragEnter event to the List. Select Generate DragDrop handler to have Flash Builder create the event handler for you. In addition to the event object automatically passed, add a second parameter of the String cartFormat. dragEnter=”cartList_dragEnterHandler(event, ’cartFormat’)”
11 Locate the newly generated event handler and remove the TODO comment. From the file Task3_Label_to_List.mxml, copy the contents (not the entire function) of the doDragEnter() function and paste it in the newly generated event handler. Also add a second parameter named format, typed as a String. Be sure your handler appears as shown here. private function cartList_dragEnterHandler(event:DragEvent,format:String):void{ if(event.dragSource.hasFormat( format )){ DragManager.acceptDragDrop( event.target as IUIComponent ); } }
This function has only one purpose: to check whether formats enable the drag initiator to be dropped. The if statement determines whether there are matching formats; then the acceptDragDrop() method allows the actual dropping to take place. 12 At the top of the Script block, import the mx.managers.DragManager and mx.core.IUIComponent classes. These classes are used in the function you just copied into the file, and therefore not automatically imported. 13 Run the FlexGrocer.mxml application. You should be able to drag the grocery item data; when you drag the pointer over the shopping cart List, you should see the red X disappear, and you can drop the drag proxy. At this point, nothing happens when you drop the drag proxy. 14 In ShoppingView.mxml, locate the List with the id of cartList and add a dragDrop event to the List. Select Generate DragDrop handler to have Flash Builder create the event handler for you. In addition to the event object automatically passed, add a second parameter of the String cartFormat. dragDrop=”cartList_dragDropHandler(event, ’cartFormat’)”
You pass the information to this function, which will place data in the shopping cart.
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15 Locate the newly generated event handler and remove the TODO comment. Also accept a second parameter in the function named format typed as String. protected function cartList_dragDropHandler(event:DragEvent, format:String):void { }
16 As the first line of code in the function, create a variable local to the function named product, typed as a Product. Assign product the result of calling the dataForFormat() method of the event.dragSource object, passing the format as an argument. You will need to cast the result as a Product. var product:Product = event.dragSource.dataForFormat( format ) as Product;
This Product object is needed to create a ShoppingCartItem in the next step of the task. The dataForFormat() method retrieves data based on the format used as the argument of the function. In this case, the data stored in the DragSource object was the product data added in step 6 of this task using the addData() method. Tip: You can open the file valueObjects/ShoppingCartItem .as and review that the constructor’s parameters are a Product object and an optional quantity .
17 Next in the function, create a variable local to the function named shoppingCartItem, typed as a ShoppingCartItem. Assign that variable equal to a new ShoppingCartItem. The arguments of the ShoppingCartItem constructor should be the Product object created in the last step and the number 1. var shoppingCartItem:ShoppingCartItem = new ShoppingCartItem(product,1);
Here is a quick review of how the Product object got in the DragSource: • In steps 4, 5 and 6 of this exercise, you passed a Product object to the img_mouseDownHandler() function. • The function placed the Product object into the DragSource object using the addData() method and associated it with the cartFormat format. • In the event handler just created, you retrieved that same Product object and will now place it in the shopping cart. 18 As the last line of code in the function, invoke the addItem() method of the shoppingCart object and pass the shoppingCartItem variable as a parameter. Check to be sure your function appears as shown here: protected function cartList_dragDropHandler(event:DragEvent, format:String):void{ var product:Product = event.dragSource.dataForFormat( format ) as Product;
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var shoppingCartItem:ShoppingCartItem = new ShoppingCartItem(product,1); shoppingCart.addItem( shoppingCartItem ); }
The method invocation actually places the ShoppingCartItem object in the shopping cart. 19 Run the application. You can now drag grocery items into the shopping cart. You see that the drag-and-drop operation is working, but the drag proxy is the default proxy for the Image. In the next step you add code so the drag proxy becomes the image of the grocery item. 20 Return to ProductItem.mxml. At the bottom of the block, locate the img_mouseDownHandler() function. At the top of the function create a variable local to the function named proxy of type BitmapImage. Assign the newly created variable a new BitmapImage object. As the next line of code in the function, assign proxy.source the value img.content. var proxy:BitmapImage = new BitmapImage(); proxy.source = img.content;
Here you are creating a new image to act as the proxy. This code may cause you to have some questions. Your first question might be, why not just use the existing Image object as the proxy? This is because by default the drag-and-drop operation removes the drag proxy from its source. You could have simply used the existing Image as the drag proxy, but after dragging and dropping, the image would no longer be shown with the other grocery item data. The next question could be, what is a BitmapImage and why use it instead of another Image instance? Actually you could have used another Image instance, but it would not be the best choice. The Image tag is quite complex and performs many functions, including retrieving an actual image. This makes the component quite “heavy.” The BitmapImage only has the ability to display data that is already loaded and is therefore a perfect fit for this implementation since the Image tag has already loaded the actual image. BitmapImage is much more svelte than the Image tag, so a better choice in this case. Finally, it may seem odd that you assign the source property of the BitmapImage the value of the content property of the Image tag. Why not just assign the source property of Image to the source property of BitmapImage? This, again, goes back to the functionality of the Image tag. You specify a source in the Image tag, which is the path to the object to load. The Image tag then places the retrieved image in the content property. The BitmapImage uses the source property as the actual image, not an object to retrieve as does the Image tag, hence proxy.source = img.content.
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21 In the DragManager.doDrag() method invocation, add a fourth parameter of proxy. DragManager.doDrag( event.target as IUIComponent, dragSource, event, proxy);
This fourth parameter represents the dragImage. Instead of the outline of the Image of the grocery item data being the drag proxy, you have now specified that the image of the item should be displayed when dragging is taking place. 22 Save the file and check the Problems view. You get the following error: Implicit coercion of a value of type spark.primitives:BitmapImage to an unrelated ➥type mx.core:IFlexDisplayObject;
The fourth parameter, the drag proxy, must implement the interface IFlexDisplayObject to be a valid drag proxy. BitmapImage does not do this, hence the error. You will solve this issue in the next step. 23 Immediately below the code you’ve just written, create a variable local to the function named imageHolder, data typed as a Group, and assign it a new Group object. Next use the addElement() method of the imageHolder Group object and pass as a parameter the proxy object. var imageHolder:Group = new Group(); imageHolder.addElement(proxy);
Be sure to either use code-completion or manually import spark.components.Group. Since the Group class does implement IFlexDisplayObject, you will place the BitmapImage in a Group so it can be used as a drag proxy. 24 In the DragManager.doDrag() method invocation, change the fourth parameter from proxy to imageHolder. DragManager.doDrag(event.currentTarget as IUIComponent, dragSource, event, ➥imageHolder);
After this change, save the file and you will see the error is no longer in the Problems view. 25 Check to be sure that your img_mouseDownHandler() function appears as follows, and then run the application. You should be able to drag the image of the grocery item and drop it in the cart. protected function img_mouseDownHandler(event:MouseEvent,format:String):void { var proxy:BitmapImage=new BitmapImage(); proxy.source=img.content; var imageHolder:Group = new Group(); imageHolder.addElement(proxy);
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var dragSource:DragSource=new DragSource(); dragSource.addData(product,format); DragManager.doDrag(event.currentTarget as IUIComponent, dragSource, event, ➥imageHolder); }
What You Have Learned In this lesson, you have: • Implemented drag-and-drop operations between two drag-enabled components and used the default drop process (pages 329–332) • Implemented drag-and-drop operations between two drag-enabled components and customized the drop process to use the data stored in the DragSource object (pages 333–336) • Implemented drag-and-drop operations between non-drag-enabled components and used a custom dragImage (pages 337–343) • Implemented drag-and-drop operations in the shopping cart (pages 343–349)
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LESSON 14
What You Will Learn In this lesson, you will: • Use the ViewStack class as the basis for implementing navigation • Learn about the NavigatorContent class, which allows Spark-based containers in a ViewStack • Use the ViewStack selectedIndex and selectedChild properties for navigation
Approximate Time This lesson takes approximately 1 hour and 30 minutes to complete.
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Lesson 14
Implementing Navigation Imperative to any application is a navigation system. Users should be able to easily move around in an application and locate the functionality they need. Flex applications can implement navigation in one of two ways: using states, as you learned in the earlier lessons, and by using navigator containers. Some navigation will be completely at the user’s discretion, such as clicking a button to move to the home page or the checkout process. Other navigation can be tightly controlled by the developer—for example, a checkout process in which users cannot proceed to the next screen until certain conditions are met on an existing screen. In this lesson, you will implement both types of navigation.
The checkout process will be controlled by a ViewStack, one of Flex’s navigator containers.
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LESSON 14: Implementing Navigation
Introducing Navigation Navigation enables users to move through your application and (just as important) enables you to control user movement through the application. You will enable both user-controlled movement and application-controlled movement in this lesson when implementing a checkout process for the e-commerce application. During this process, you need to control which screens users see and when they see them. A navigator container is a container that implements one specific rule on positioning its children: Only one child can ever be seen at the same time. This allows for a multipage view within the application. To help visualize how this works, picture a stack of papers on your desk. At any time, only one page in that stack can be on top, and therefore visible. To do this, the ViewStack toggles the visibility of its children, always ensuring that one and only one child can be seen at any time. The Navigation components are still implemented using the mx architecture, meaning they are the same components that existed in Flex 3. To enable developers to use both Spark and MX components as children of a ViewStack, the Flex SDK added an INavigatorContent interface. Any class that implements INavigatorContent can be used as a direct child of a ViewStack. In the Spark component set, there is a new container named NavigatorContent that implements the INavigatorContent interface and subclasses the Spark Group class. You can use NavigatorContent instead of Group for adding Spark elements as children of the ViewStack. Although the ViewStack is the key element to implementing navigation in Flex, it does not intrinsically have a way to switch which child is visible; that must be done using another tool. You can use built-in tools to control the ViewStack or build your own. NoTe: Although ViewStack does not have any visible way for a user to navigate between the views, the Accordion and TabNavigator implement the same functionality as a ViewStack, but also provide user interface controls to allow the user to navigate between the views .
To control which child is shown in a navigator container, you can interact with the container’s selectedChild or selectedIndex property. You use the selectedIndex property to choose which child of the ViewStack should be displayed. NoTe: The ViewStack is zero indexed, so the “first” child is numbered 0 . Use the selectedChild property if you would rather indicate which child of the ViewStack should be displayed by referencing the name (id) of the child rather than the numeric index. The selectedChild property will display the appropriate container in the ViewStack based
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on the instance name provided in the id property. The following example shows how to use plain Button components to control which child of the ViewStack is displayed using both selectedChild and selectedIndex:
When run, this code creates the result shown in the following figure, where a ViewStack is used with buttons.
With this brief overview of the navigator containers, you are now ready to implement navigation in your applications.
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LESSON 14: Implementing Navigation
Creating the Checkout Process as a ViewStack In this exercise, you will import three views that will comprise the checkout process, and add a view stack to allow the user to navigate between them. 1 Ensure you have your FlexGrocer project open. Alternatively, if you didn’t complete the previous lessons or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson14/start folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Import the Checkout.fxp project from the Lesson14/independent folder. A few classes are already provided for you. You will be copying these classes into your application.
Run the Checkout application. Notice there are three pages of forms that you can navigate between by clicking the Proceed button. 3 In the checkout project, select the events, valueObjects, and views.checkout packages, and copy them to the clipboard (Ctrl-C/Command-C). In the FlexGrocer project, select the src directory, and paste the copied folders. A warning box appears, telling you that the events folder already exists. Flex asks if you want to merge the folder’s contents and overwrite any existing files with the same names. Click Yes To All.
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The merged project after the paste should look like this:
If your views package doesn’t have a checkout package inside it, you should undo and try again. NoTe: Some new constructs have been introduced in the views .checkout classes that will be explained in more detail in Lesson 15, “Using Formatters and Validators .”
4 Right-click the Checkout project and select Close Project. 5 Right-click the FlexGrocer views package and choose New MXML Component. Give the new component the name CheckOutView, select spark.layout.VerticalLayout for Layout, base the component on spark.components.Group, and remove the preset height and width attributes. 6 In the Declarations block, add an instance of the OrderInfo class, which exists in the valueObjects package, and specify an id of orderInfo.
If a namespace isn’t automatically created for the valueObjects package, add one to the root node like this: xmlns:valueObjects=”valueObjects.*”
7 After the closing the Declarations tag, add a ViewStack (from the mx namespace) that has an id of vs, and a height and width of 100% .
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8 Add an instance of the CustomerInfo class (which exists in the views.checkout package), as a child of the ViewStack by adding it inside of the opening and closing ViewStack tag. Bind the local orderInfo object to the orderInfo property for the CustomerInfo class, and set its height and width to 100% .
If a namespace isn’t automatically added to the root node for the checkout package, manually add one like this: xmlns:checkout=”views.checkout.*”
If you look at the source of CustomerInfo, you will see that its root node is NavigatorContent, so it is allowable as a child of the ViewStack. Additionally, you will find that it has a public bindable property called orderInfo, which is bound to the controls. 9 Create a second child of the ViewStack, as an instance of the CreditCardInfo class, which exists in the views.checkout package. Bind the local orderInfo object to the orderInfo property for the CreditCardInfo class, and set its height and width to 100% .
Much like the CustomerInfo class, CreditCardInfo has a root node of NavigatorContent, making it allowable as a child of the ViewStack. It also has a public bindable property called orderInfo, which is bound to the controls. 10 Create the third and final child of the ViewStack, as an instance of the Review class from the views.checkout package. Bind the local orderInfo object to the orderInfo property for the Review class, and set its height and width to 100% .
Much like the CustomerInfo and CreditCardInfo classes, Review also has a root node of NavigatorContent, making it allowable as a child of the ViewStack. It also has a public bindable property called orderInfo, which is bound to the controls. 11 Open the CustomerInfo, CreditCardInfo, and Review classes. Notice that all three broadcast one or more events when you click the buttons in those classes. Next you need to add handlers for those events. 12 In CheckOutView.mxml, add an event handler for the proceed event from both the CustomerInfo and CreditCartInfo classes, which will call a soon-to-be-written function called handleProceed() and pass the implicit event object as an argument.
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The code in the ViewStack should read like this:
Next you will need to write the handleProceed() method. 13 Add an block between the end of the Declarations block and the start of the ViewStack. Within the Script block, add a private method called handleProceed(), which accepts an argument named event as an instance of the flash.events.Event class and a void return type. The body of the function should increment the selectedIndex of the ViewStack by 1. private function handleProceed( event:Event ):void { vs.selectedIndex = vs.selectedIndex + 1; }
This method allows users to navigate from the CustomerInfo to the CreditCardInfo to the Review views of the ViewStack. Whether they are in the CustomerInfo or the CreditCardInfo view, clicking the Continue button will bring them to the next screen. 14 Still in the Script block, add a method called handleEdit(), which accepts an event as an instance of the Event class and a void return type. The body of this method should set the selectedIndex of the ViewStack back to element 0. private function handleEdit( event:Event ):void { vs.selectedIndex = 0; }
This method will be used if the user clicks the Edit Information button on the Review screen. 15 Add an event listener on the Review instance for the editInformation event that will call your handleEdit() method:
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16 Add one more event handler on the Review instance for the completeOrder event that will call a soon-to-be-written handleComplete() method.
17 In the Script block, add a new private method called handleComplete() that accepts an event as an argument and a void return type. Inside this method, set the selectedIndex of the ViewStack back to 0, and dispatch a new OrderEvent with the name placeOrder that uses the orderInfo property as the order for the event. Finally, reset the orderInfo property to be a new OrderInfo instance. private function handleComplete( event:Event ):void { //reset the navigation for the next order vs.selectedIndex = 0; //Send the info up to the event chain dispatchEvent( new OrderEvent( ‘placeOrder’ , orderInfo ) ); //Clear out this order object for next time orderInfo = new OrderInfo(); }
Make sure that you have an import statement for the OrderEvent class from the events package. 18 After the closing Declarations tag but before the opening Script tag, add metadata to the CheckoutView class declaring that this class is capable of broadcasting an event called placeOrder of the type OrderEvent. [Event(name=”placeOrder” , type=”events.OrderEvent”)]
19 Add a public bindable property to the CheckoutView class called shoppingCart as an instance of the ShoppingCart class. import cart.ShoppingCart; [Bindable] public var shoppingCart:ShoppingCart;
As always, ensure that you import the ShoppingCart class.
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20 Bind the local shoppingCart property into the Review class instance’s shoppingCart property.
This will allow a shoppingCart property to be passed from the main application into the CheckoutView, and then into the Review class itself. 21 Save and close CheckoutView. 22 Open ShoppingView.mxml. Find where the shoppingCart is declared and instantiated. Remove the instantiation of the new ShoppingCart. Save and close ShoppingView.mxml. [Bindable] public var shoppingCart:ShoppingCart;
Since you need to share a single shoppingCart between the shopping and checkout view, you will create the single instance in the main application and pass a reference to each. All that remains now is to allow the rest of the application to effectively use this new CheckoutView you have created.
Integrating CheckoutView into the Application Now that you have a class to handle the checkout process, you need to integrate it into the rest of the application. 1 Open FlexGrocer.mxml and close all other files. Find the label with the copyright information, and move it to after the instantiation of the ShoppingView component.
Having this label defined after the other views will ensure it will always be seen on top of anything else in the application.
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2 Between the ShoppingView and Label components, create an instance of the CheckoutView component. Give it an x and y value of 0, and a height and width of 100% .
As you save the application now, you should have your first chance to find any typos in the CheckoutView class you created. If the application compiles without error, continue. If not, check the code you created in the CheckOutView and fix any errors before proceeding. 3 Create a states block in the FlexGrocer application, which contains two states: one called shoppingView, and the other called checkoutView.
4 Set the ShoppingView instance to be included only in the shoppingView state, and the CheckoutView instance to be included only in the checkoutView state.
5 In the controlBarContent node, add includeIn=”shoppingView” to the List instance.
6 In the Script block, add two new methods. The first, handleCheckoutClick(), accepts a MouseEvent as an argument, has a void return type, and sets the currentState to checkOutView. The second, handleFlexGrocerClick(), accepts a MouseEvent as an argument, returns void, and sets the currentState to shoppingView. private function handleCheckoutClick( event:MouseEvent ):void { this.currentState = “checkoutView”; }
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private function handleFlexGrocerClick( event:MouseEvent ):void { this.currentState = “shoppingView”; }
7 Add a click handler on the btnCheckout button, which calls the handleCheckoutClick() method and passes the event object. Also add a click handler to the Flex Grocer button that calls the handleFlexGrocerClick() method and passes an event object. 8 In the Declarations section, add a ShoppingCart instance with an id of shoppingCart.
As always, ensure you add the proper namespace to the root node. 9 Bind shoppingCart to the public shoppingCart properties of both the ShoppingView and CheckOutView components. This will allow a single instance of the shoppingCart to be used by both the views.
10 Add a private method to the Script block called handlePlaceOrder() that accepts an instance of the OrderEvent class as an argument and returns void. This method will use an Alert to display information about the order, and will then empty the cart by assigning it a new instance of the shoppingCart class and set the currentState back to shoppingView. private function handlePlaceOrder( event:OrderEvent ):void { //send the data to the server somewhere Alert.show( "Placed an order for " + event.order.billingName + " ➥with a total of " + shoppingCart.total, "Order Placed" ); //empty the shoppingCart shoppingCart = new ShoppingCart(); //Send the user back to the shoppingView currentState = "shoppingView"; }
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LESSON 14: Implementing Navigation
The Alert class has a static method named show. When you call this method and pass it a string, Flex will show an Alert box with your message. If you had a server to submit the order to, the call to that server-side method would go here. You can learn about that in Volume 2 of this book. For now, use an Alert box to display information about the order. As always, ensure that the events.OrderEvent and mx.controls.Alert classes are properly imported. 11 Add an event handler for the placeOrder event, which calls your newly written handlePlaceOrder() method and passes the event object to it.
Save your files and run the application. You should now be able to add elements to the cart, click the checkout button, navigate through the checkout process, and complete the order. The application then resets the cart and returns you to the shopping experience.
What You Have Learned In this lesson, you have: • Used a ViewStack to allow users to navigate the checkout process (pages 354–357) • Learned about the NavigatorContent class, which allows Spark containers in a ViewStack (pages 352–353) • Controlled navigation with the ViewStack’s selectedIndex and selectedChild properties (pages 357–358)
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From the Library of Wow! eBook
LESSON 15
What You Will Learn In this lesson, you will: • Use a formatter and remove string concatenation • Look at a two-way data binding example • Use a validator to check if data is in a valid format • Learn to trigger validation in ActionScript
Approximate Time This lesson takes approximately 1 hour to complete.
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Lesson 15
Using Formatters and Validators Flex provides many types of built-in formatters and validators that enable you to display and validate user-supplied data such as dates, numbers, and currencies. Using the built-in data validators on the client side, you can make your application perform better by reducing calls to the server for validation. You can also save development time by using the built-in formatters to automate the often repetitive process of formatting data.
Validating customer information
365 From the Library of Wow! eBook
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LESSON 15: Using Formatters and Validators
Introducing Formatters and Validators Flex formatters convert raw data into a customized string using predefined rules. The formatters can be used in MXML or in ActionScript and work well with data binding to simplify the display of formatted data. Validators are used to ensure that data meets specific criteria before the application attempts to use it. This can be particularly important if you expect a user to input a number for a mathematical operation or a date for scheduling. Like formatters, validators can be used in MXML or ActionScript. They provide logical feedback on data input (valid or invalid) but also provide visual feedback in the way of red borders and error messages when input is invalid.
Formatters A formatter is simply an ActionScript class that descends from (is a subclass of) the Formatter class. Some of the formatters available include: • mx.formatters.CurrencyFormatter • mx.formatters.DateFormatter • mx.formatters.NumberFormatter • mx.formatters.PhoneFormatter • mx.formatters.ZipCodeFormatter Formatters manage quite a bit of complexity for you, but they are exceedingly simple to use. Here is a CurrencyFormatter defined in MXML:
This formatter can be applied either in ActionScript or in MXML with data binding, using the following syntax: trace( myFormatter.format( 123 ) ); //outputs $123.00 in the United States
In the latter example, each time someData changes, the format() method will be called and the output displayed in the label.
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Introducing Formatters and Validators
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Previously, you accomplished something similar using concatenation of strings. You wrote code like this:
This strategy has several problems. First, it becomes complicated to control variables defining how the user wants to see this currency presented. For example, if you are creating a globalized application, you will need to support different currency symbols, different regional uses of commas and periods, and varying degrees of precision. Second, this code assumes that the currency symbol will always appear before the number. This is certainly not the case in many countries. By using Flex formatters, these and other issues are handled for you.
Validators Flex also has a set of Validator classes that you can use to check whether a data type is valid (for example, if the input is a number) and to ensure that the input has been formatted correctly (for example, if a date is entered in a specific format). As with Formatter classes, you can use Validator classes either as MXML tags or instantiate them in ActionScript. Using validators, you can perform a large amount of data validation in the client application, instead of waiting until data is submitted to the server. Not only does this provide a more responsive user experience, it also reduces the number of calls between the client and the server. This yields a better-performing application. Client-side validation is not a perfect solution; some types of data validation (such as security issues) are still best performed at the server. But using Validator classes at the client reduces server calls to only these use cases. All Flex validators are a subclass of the Validator class. Some of the validators available as part of the Flex framework include: • CreditCardValidator • DateValidator • EmailValidator • NumberValidator • PhoneNumberValidator • SocialSecurityValidator • StringValidator • ZipCodeValidator
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LESSON 15: Using Formatters and Validators
Much like the Formatter classes, the Validator classes cover a large number of use cases and conditions that you might not consider on your own. By using the Flex validators, you are better prepared for robust applications and internationalization requirements.
Using Formatter Classes In this exercise, you will apply a CurrencyFormatter class so all the price selections are displayed as local currency in the FlexGrocer application. There are multiple places in which prices are displayed in the application, including: • The list of grocery products displayed • The total of the shopping cart • The subtotal and list prices in the user’s shopping cart • The checkout process The CurrencyFormatter adjusts the decimal rounding and precision and sets the thousands separator and the negative sign. You can specify the type and placement of the currency symbol used, which can contain multiple characters, including blank spaces. 1 Open ShoppingView.mxml from your views package. Alternatively, if you didn’t complete the previous lesson or your code is not functioning properly, you can import the FlexGrocer.fxp project from the Lesson15/start folder. Please refer to Appendix A for complete instructions on importing a project should you ever skip a lesson or if you ever have a code issue you cannot resolve. 2 Within the tags, add an tag. Assign the tag an id of currency and specify a precision of 2:
The decimal rounding, thousands separator, currency symbol, and the negative sign are properties that can be set on the CurrencyFormatter; we have left these at their defaults. You specified a precision of 2, meaning that two decimal places will always be displayed. The CurrencyFormatter receives its default settings from user locale information. This locale information exists inside resource bundles that you can create for various locales; however, doing so is beyond the scope of this book.
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Using Formatter Classes
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NoTe: If you would like to learn more about resource bundles or the process of internationalizing an application, refer to “Creating Resources” in the Adobe Flex 4 help documentation .
3 Locate the Label control that displays the words Your Cart Total along with a dollar sign and the shopping cart’s total. Inside the data binding for the text property, replace the dollar sign and binding expression with a call to the format() method of the currency object, and pass shoppingCart.total to the method, as follows:
